fix(session): return METHOD_NOT_FOUND for unknown request methods

[Ranga-Prasath-22]

Summary

Return JSON-RPC -32601 METHOD_NOT_FOUND for request methods that are not part of the session's request schema, while preserving -32602 INVALID_PARAMS for known methods with invalid parameters. Error responses echo the request id unchanged.

Why

BaseSession previously mapped every request-model validation failure to -32602. That made an unsupported method indistinguishable from a malformed call and caused clients to retry requests that could never succeed.

Implementation

• Derive the supported request-method set from the configured Pydantic request type once per session.
• Apply the distinction centrally in BaseSession, covering both client and server sessions and custom request unions.
• Fail closed if schema introspection cannot produce a method set.

Behavior note

Validation-error responses no longer include the empty data="" field (-32602 and the new -32601 paths both omit it). This is spec-legal — the JSON-RPC data member is optional and its type is implementation-defined — and matches the TypeScript SDK, but clients inspecting error.data may notice the change.

Tests

• Symmetric regression coverage for unknown methods and malformed known methods on both client and server sessions.
• Unknown notifications are verified to produce no response, per JSON-RPC requirements.
• Direct assertion that _extract_known_request_methods extracts the full expected method set for ClientRequest/ServerRequest, plus fallback behavior for non-string defaults and introspection failures.

Validation

• Focused session tests pass (tests/shared/test_session.py, tests/server/test_session.py).
• Ruff format and Ruff lint pass.
• Behavior verified against the earlier review comments on this PR.

Fixes #3193
Resolves #1561

[Ranga-Prasath-22]

Just a quiet follow-up with some verification evidence, no rush -- I know the queue is deep.

Status: this PR makes the session return JSON-RPC -32601 (METHOD_NOT_FOUND) for unknown request methods instead of -32602 (INVALID_PARAMS).

Repro (stdlib + SDK only, anyio memory streams against ServerSession):

# send an unknown-method request to a ServerSession
await client_to_server_send.send(SessionMessage(types.JSONRPCMessage(
    types.JSONRPCRequest(jsonrpc="2.0", id=1, method="totally/bogus", params={}))))
resp = await server_to_client_receive.receive()
print(resp.message.root.error.code, resp.message.root.error.message)

Observed (same script, pip install -e ., one checkout per branch):

checkout	error code	message
v1.x (98b7159)	-32602	Invalid request parameters
PR head (9936777)	-32601	Method not found

Also checked for regressions: a known method with malformed params (initialize with protocolVersion: 12345) still returns -32602 on the PR head, so classification only affects unknown methods.

Happy to add docs or adjust the error message wording if that's the blocker -- could I get a review pass?

[Ranga-Prasath-22]

Follow-up: fix the pre-commit (pyright) failure

Root cause. checks / pre-commit failed because pyright reported reportUnnecessaryCast at src/mcp/shared/session.py:178: cast(Any, t.model_fields["method"].default) is redundant — t is typed Any, so .default is already Any, making the cast(Any, …) a no-op that pyright flags.

Fix. Drop the cast(Any, …) and the now-unused cast import.

Verification.

• pre-commit run --all-files (local): end-of-file-fixer, prettier, markdownlint, ruff-format, and ruff all pass; pyright now reports 0 errors on the PR's files. (The remaining local pyright noise is Windows-only analysis of the posix/win32 mcp.os modules in untouched files, which does not occur on the ubuntu CI runner.)
• ruff format/check on session.py: clean.
• pytest tests/server/test_session.py tests/shared/test_session.py -q: 19 passed.