fix(mcp): handle ping requests

[loveRhythm1990]

Summary

• implement the MCP ping method for stdio and Streamable HTTP transports
• return an empty result for ping requests and HTTP 204 for notifications
• keep authentication and rate limiting while excluding ping heartbeats from API call logs and usage statistics
• centralize RPC method parsing so supported methods are consistent across transports

Validation

• cargo test -p memoria-mcp --lib
• cargo test -p memoria-api --lib
• cargo test -p memoria-api --test api_e2e test_mcp_ping --no-run
• cargo clippy -p memoria-mcp --lib -- -D warnings
• cargo clippy -p memoria-api --lib -- -D warnings

Closes #235

[aptend]

Reviewed the ping handling across stdio and Streamable HTTP. The request/notification responses follow JSON-RPC semantics, while authentication and rate limiting remain outside the short-circuit. Ping bypasses tool usage and API-call accounting as intended. No blocking issues; all checks are green.