refactor(testing): inline networking decode-failure dispatch (leanEthereum#1156)

Replace the module-level decoder lookup table with a match in its sole
consumer. The table mapped seven names to already-imported decoder
callables and was only ever read by the decode-failure attempt, so the
dispatch now lives where it is used.

- Delete the _DECODERS_BY_NAME table and its now-orphaned Callable and
  Final imports.
- Dispatch by structural pattern matching on the decoder discriminator.
- Keep the hex decode inside the try, so malformed-hex input is still
  caught and reported as the rejection.

Pure structural refactor: emitted vectors are byte-identical (verified by
filling tests/consensus/lstar/networking before and after).

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: tcoratger

packages/testing/src/consensus_testing/test_fixtures/networking_codec.py

@@ -1,7 +1,6 @@
 """Networking codec test fixture for wire-format conformance testing."""
 
-from collections.abc import Callable
-from typing import Annotated, ClassVar, Final, Literal
+from typing import Annotated, ClassVar, Literal
 
 from pydantic import Field
 
@@ -654,18 +653,6 @@ class DecodeFailureOutput(StrictBaseModel):
     """Name of the decoder that must reject the input."""
 
 
-_DECODERS_BY_NAME: Final[dict[str, Callable[[bytes], object]]] = {
-    "varint": decode_varint,
-    "snappy_frame": frame_decompress,
-    "snappy_block": decompress,
-    "gossipsub_rpc": RPC.decode,
-    "reqresp_request": decode_request,
-    "reqresp_response": ResponseCode.decode,
-    "enr": ENR.from_rlp,
-}
-"""Wire-format decoders keyed by the name a rejection vector targets."""
-
-
 class DecodeFailure(StrictBaseModel):
     """Assert that a wire-format decoder rejects malformed input."""
 
@@ -689,7 +676,22 @@ class DecodeFailure(StrictBaseModel):
     def attempt_decode(self) -> Exception | None:
         """Run the decoder on the malformed input and return what it raised."""
         try:
-            _DECODERS_BY_NAME[self.decoder](from_hex(self.raw_bytes))
+            raw_bytes = from_hex(self.raw_bytes)
+            match self.decoder:
+                case "varint":
+                    decode_varint(raw_bytes)
+                case "snappy_frame":
+                    frame_decompress(raw_bytes)
+                case "snappy_block":
+                    decompress(raw_bytes)
+                case "gossipsub_rpc":
+                    RPC.decode(raw_bytes)
+                case "reqresp_request":
+                    decode_request(raw_bytes)
+                case "reqresp_response":
+                    ResponseCode.decode(raw_bytes)
+                case "enr":
+                    ENR.from_rlp(raw_bytes)
         except Exception as exception:
             return exception
         return None