test(clear_signing): use jsons for testing external defintions

### common/tests/fixtures/ethereum/definitions/eth/chain-id/1/README.md
@@ -1,29 +1,74 @@
 # External Definitions Test Fixtures
 
-Directory for encoded (dev) signed token definitions and ERC-7730 contract
-descriptors, used for device testing with external definitions. These files
-back the fixtures in [`sign_tx_external_definitions.json`](../../../../sign_tx_external_definitions.json).
+Directory for network and token definitions and ERC-7730 contract descriptors,
+stored as JSON and used for device testing with external definitions. These
+files back the fixtures in [`sign_tx_external_definitions.json`](../../../../sign_tx_external_definitions.json).
 
-## Updating the definitions
+## Format
 
-Keep this directory up to date with the latest definitions from the
-`definitions.tar.xz` or `deploy.tar.xz` tarballs, that can be obtained from the [`trezor/definitions`](https://github.com/trezor/trezor-common-definitions)
-repo by running:
+Each file holds the definition type and the decoded protobuf message (bytes as
+hex, enums by name):
+
+```json
+{
+  "data_type": "ETHEREUM_NETWORK",
+  "message": {
+    "chain_id": 1,
+    "symbol": "ETH",
+    "slip44": 60,
+    "name": "Ethereum"
+  }
+}
+```
+
+The tests serve them through `JsonSource` (in `tests/definitions.py`), which
+encodes and dev-signs each definition when the device requests it, always with
+format version 2 and the maximum timestamp, so the fixtures never expire.
+
+The files are arranged as:
+
+```
+definitions/eth/chain-id/<chain-id>/network.json
+definitions/eth/chain-id/<chain-id>/token-<token-address>.json
+definitions/eth/chain-id/<chain-id>/display-format/<contract-address>-<function-signature>.json
+```
+
+## Adding definitions
+
+Generate the definitions with the [`trezor/definitions`](https://github.com/trezor/trezor-common-definitions)
+repo, in an external directory:
 
 ```sh
 python cli.py generate --dev-sign -o <output_folder>
 ```
 
-Run the command above in an external directory, then pick the contract call
-you want to test.
+(Or you can ask the last release manager for the latest definitions or the
+ERC-7730 FW maintainer(s).)
 
-(Or you can ask the last release manager for the latest definitions or the ERC-7730 FW maintainer(s))
+Pick the `.dat` files for the contract calls you want to test, and convert
+each one to JSON next to it:
 
-The generated files are arranged as:
+```python
+import io, json, sys
+from pathlib import Path
 
+from trezorlib import definitions, messages, protobuf
+
+TYPES = {
+    messages.DefinitionType.ETHEREUM_NETWORK: messages.EthereumNetworkInfo,
+    messages.DefinitionType.ETHEREUM_TOKEN: messages.EthereumTokenInfo,
+    messages.DefinitionType.ETHEREUM_DISPLAY_FORMAT: messages.EthereumDisplayFormatInfo,
+}
+
+for name in sys.argv[1:]:
+    path = Path(name)
+    payload = definitions.Definition.parse(path.read_bytes()).payload
+    msg = protobuf.load_message(io.BytesIO(payload.data), TYPES[payload.data_type])
+    converted = {"data_type": payload.data_type.name, "message": protobuf.to_dict(msg)}
+    path.with_suffix(".json").write_text(json.dumps(converted, indent=2) + "\n")
 ```
-definitions/eth/<chain-id>/<display-format>/<contract-address>-<function-signature>.dat
-```
+
+Then copy the `.json` files here, keeping the layout above.
 
 ## Important
 

### tests/definitions.py
@@ -17,8 +17,10 @@
 from __future__ import annotations
 
 import io
+import json
 import typing as t
 from hashlib import sha256
+from pathlib import Path
 
 from trezorlib import cosi, definitions, messages, protobuf
 from trezorlib.testing.common import PRIVATE_KEYS_DEV
@@ -59,13 +61,7 @@ def make_payload(
     format_version: bytes = b"2",
     data_type: messages.DefinitionType = messages.DefinitionType.ETHEREUM_NETWORK,
     timestamp: int = 0xFFFF_FFFF,
-    message: (
-        messages.EthereumNetworkInfo
-        | messages.EthereumTokenInfo
-        | messages.SolanaTokenInfo
-        | messages.EthereumDisplayFormatInfo
-        | bytes
-    ) = make_eth_network(),
+    message: protobuf.MessageType | bytes = make_eth_network(),
 ) -> bytes:
     if isinstance(message, bytes):
         message_bytes = message
@@ -213,3 +209,38 @@ def encode_eth_display_format(
     )
     proof, signature = sign_payload(payload, [])
     return payload + proof + signature
+
+
+_MESSAGE_TYPES: dict[messages.DefinitionType, type[protobuf.MessageType]] = {
+    messages.DefinitionType.ETHEREUM_NETWORK: messages.EthereumNetworkInfo,
+    messages.DefinitionType.ETHEREUM_TOKEN: messages.EthereumTokenInfo,
+    messages.DefinitionType.SOLANA_TOKEN: messages.SolanaTokenInfo,
+    messages.DefinitionType.ETHEREUM_DISPLAY_FORMAT: messages.EthereumDisplayFormatInfo,
+}
+
+
+def json_to_dat(d: dict[str, t.Any]) -> bytes:
+    """Encode and dev-sign a JSON definition.
+
+    Always uses format version 2 and the maximum timestamp (the `make_payload`
+    defaults), so the test definitions never expire."""
+    data_type = messages.DefinitionType[d["data_type"]]
+    payload = make_payload(
+        data_type=data_type,
+        message=protobuf.dict_to_proto(_MESSAGE_TYPES[data_type], d["message"]),
+    )
+    proof, signature = sign_payload(payload, [])
+    return payload + proof + signature
+
+
+class JsonSource(definitions.Source):
+    """Serves JSON definitions as freshly dev-signed .dat blobs."""
+
+    def __init__(self, root: Path) -> None:
+        self.root = root
+
+    def fetch_path(self, *components: str) -> bytes | None:
+        path = self.root.joinpath(*components).with_suffix(".json")
+        if not path.exists():
+            return None
+        return json_to_dat(json.loads(path.read_text()))

### tests/device_tests/ethereum/test_signtx.py
@@ -18,20 +18,18 @@
 
 import typing as t
 from itertools import product
-from pathlib import Path
 
 import pytest
 
 from trezorlib import ethereum, exceptions, messages, models
 from trezorlib.debuglink import DebugSession as Session
 from trezorlib.debuglink import message_filters
-from trezorlib.definitions import FilesystemSource
 from trezorlib.exceptions import TrezorFailure
 from trezorlib.protobuf import MessageType
 from trezorlib.tools import parse_path, unharden
 
-from ...common import parametrize_using_common_fixtures
-from ...definitions import encode_eth_network
+from ...common import COMMON_FIXTURES_DIR, parametrize_using_common_fixtures
+from ...definitions import JsonSource, encode_eth_network
 from ...input_flows import (
     InputFlowConfirmAllWarnings,
     InputFlowEthereumSignTxData,
@@ -119,13 +117,11 @@ def _do_test_signtx(
     assert sig.v == result["sig_v"]
 
 
-# Directory of dev-signed Ethereum definitions (network / token / clear-signing
-# display formats), laid out as eth/chain-id/<n>/... exactly like the deploy
-# tarball you would pass to `trezorctl ethereum --definitions <dir> sign-tx ...`.
-# Curated to only the definitions the cases below actually pull; drop more .dat
-# files in to clear-sign more contracts/functions.
-_DEFINITIONS_DIR = Path(__file__).parent / "definitions"
-_DEFINITIONS_SOURCE = FilesystemSource(_DEFINITIONS_DIR)
+# Directory of Ethereum definitions (network / token / clear-signing display
+# formats) as JSON, laid out as eth/chain-id/<n>/... like the deploy tarball.
+# `JsonSource` dev-signs each one on request. Curated to only the definitions
+# the cases below actually pull; add more to clear-sign more contracts/functions.
+_DEFINITIONS_SOURCE = JsonSource(COMMON_FIXTURES_DIR / "ethereum" / "definitions")
 
 
 @parametrize_using_common_fixtures("ethereum/sign_tx_external_definitions.json")