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.

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.
• Add symmetric regression coverage for unknown methods, malformed known methods, client-side handling, non-string schema defaults, and introspection fallback.

Validation

• 17 focused session tests pass.
• Ruff format and Ruff lint pass.
• Full CI coverage remains enforced at 100%.

Fixes #3193
Resolves #1561