Fixes #1561.

Summary

• return METHOD_NOT_FOUND for unknown JSON-RPC request methods instead of collapsing them into the generic request-validation INVALID_PARAMS fallback
• keep INVALID_PARAMS for malformed known methods by only special-casing method names that are outside the typed client request union
• add a focused regression proving invalid/method returns -32601 while malformed initialize still returns -32602

Testing

• PYTHONPATH=src python -m pytest -o addopts="" tests/issues/test_1561_invalid_method_names.py -q
• python -m ruff check src/mcp/shared/session.py src/mcp/server/session.py tests/issues/test_1561_invalid_method_names.py
• git diff --check