feat(server): convert ping messages to be spec compliant

[rwjblue-glean]

Replace string ping messages system added in #80 with MCP spec compliant JSON-RPC requests.

• Update request handler to ignore ping responses (add Result field to baseMessage to disambiguate empty responses from other incoming messages)
• Implement MCP spec compliant ping request

Summary by CodeRabbit

• New Features

  • Added support for sending JSON-RPC "ping" requests as keep-alive messages over server-sent events (SSE).
  • Enhanced the server to explicitly recognize and ignore response messages to previously sent requests.

• Bug Fixes

  • Improved handling of incoming messages to prevent unintended processing of response messages.

• Tests

  • Added tests to verify that clients receive and can respond to JSON-RPC "ping" messages over SSE.

[coderabbitai]

Walkthrough

This change updates the server's keep-alive mechanism for Server-Sent Events (SSE) by replacing the previous comment-style ping with a JSON-RPC 2.0 "ping" request that includes an incrementing request ID. The server now sends these structured ping requests, and the request handler is updated to recognize and ignore incoming JSON-RPC responses. A new test verifies that clients can receive these ping messages and respond appropriately, ensuring the server correctly processes the responses.

Changes

File(s)	Change Summary
server/request_handler.go	Added a Result field to the baseMessage struct and updated HandleMessage to detect and ignore JSON-RPC response messages.
server/sse.go	Added an atomic.Int64 requestID field to sseSession and replaced comment-style pings with JSON-RPC "ping" requests containing incrementing IDs.
server/sse_test.go	Added a subtest to TestSSEServer that verifies client receipt and response to JSON-RPC "ping" messages.

Possibly related PRs

• feat: add ping for sse server #80: Introduced the initial SSE keep-alive ping mechanism, which is directly extended and refactored in this PR to use JSON-RPC requests and handle responses.

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📥 Commits

Reviewing files that changed from the base of the PR and between 8d4be5e and 3aa0aea.

⛔ Files ignored due to path filters (1)

• server/internal/gen/request_handler.go.tmpl is excluded by !**/gen/**

📒 Files selected for processing (3)

• server/request_handler.go (2 hunks)
• server/sse.go (2 hunks)
• server/sse_test.go (2 hunks)

🚧 Files skipped from review as they are similar to previous changes (3)

• server/request_handler.go
• server/sse.go
• server/sse_test.go

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