[kingaly111]

Add rpcv2Json codegen, protocol tests and duplex integration test

Summary

Generated clients can now target services that use smithy.protocols#rpcv2Json. The runtime protocol landed in #902; this PR adds the codegen that selects it, runs Smithy's rpcv2Json protocol test suite against a generated client, and adds an end-to-end event-streaming test over real HTTP/2.

Codegen

• Rpcv2JsonProtocolGenerator and Rpcv2JsonIntegration: copies of the rpcv2Cbor versions.
  • The generator applies to services with @rpcv2Json.
  • It reads http and eventStreamHttp from the trait.
  • Generated clients default to AsyncRpcV2JsonClientProtocol and depend on smithy-http[json].
  • The sync client gets RpcV2JsonClientProtocol automatically through the existing async-to-sync mapping, so no extra runtime type was needed.
• Registration: the integration is listed in META-INF/services/...PythonIntegration, and RuntimeTypes.RPC_V2_JSON_CLIENT_PROTOCOL is added.
• SchemaTraitFilterIndex: the comments on the kept originalShapeId trait now say it's used for unit detection by both rpcv2 protocols. No behavior change.
• bigDecimal test values: ValueNodeVisitor wrote bigDecimal values in generated tests as Python float literals, which lose precision past about 17 digits and are the wrong type. It now writes Decimal("...") using the exact text from the model. This fixed 8 rpcv2Json arbitrary-precision tests and changed no other snapshot.

Protocol tests

• New projection: rpcv2-json in codegen/protocol-test/smithy-build.json, for smithy.protocoltests.rpcv2Json#RpcV2JsonProtocol from smithy-protocol-tests 1.74.1.
• Snapshot: the generated client is committed as codegen-output/rpcv2-json.
• Results: 70 passed, 4 xfailed. The 4 xfails are the explicit-default tests, which rpcv2Cbor and RestJson already skip. Generated models can't tell a member left unset from one set to its default value.

Duplex event-stream integration test (Simon Says)

• New model: my-handwritten-json-model.smithy is an @rpcv2Json copy of the hand-written event-stream service. Its projection generates AsyncMyHandwrittenJsonModel. The shapes are local copies, so the CBOR service and its snapshot are untouched.
• Shared game: the Simon Says game moved into tests/integration/_simon_says.py. Each protocol's test file supplies its client, models, codec, event content type and Smithy-Protocol value.
  • test_rpcv2cbor_event_stream.py keeps its test id and behavior.
  • test_rpcv2json_event_stream.py plays the same game using the client's own JSON codec.
  • It also checks that the client's codec has the rpcv2Json settings.
• Stronger checks, for both protocols: the server must receive initial-request followed by exactly one move or pass per turn, in order, and every frame must carry the protocol's content type.
• HTTP/2 test server:
  • H2BidiServer now takes the event content type and the Smithy-Protocol value, defaulting to rpcv2Cbor.
  • It records each frame's content type.
  • It keeps the connection open until the client ends its stream. Before, it closed as soon as the game ended, so the client's final close raised AWS_ERROR_HTTP_STREAM_HAS_COMPLETED. That's gone for both protocols.

Testing

• make generate-protocol-tests: only the new rpcv2-json and my-handwritten-json-model directories differ; every existing snapshot is byte-identical. Generation was repeated and produced the same output.
• make test-protocols: all projections pass, including rpcv2-json (70 passed, 4 xfailed) and rpcv2-cbor (69 passed, 4 xfailed).
• make typecheck-protocols: 0 errors in every projection.
• make test-functional: both test_simon_says_duplex tests and test_client_uses_rpcv2json_codec run (none skipped) and pass.
• make static-analysis, uv run pyright packages and make check-java: pass.

Notes

• No runtime package changed, so there are no changelog fragments. Codegen-only PRs in this repo don't add them.
• Regenerating snapshots needs pandoc 3.8.2, the version CI pins.