Evict streamable HTTP sessions after failed keep-alive pings

[lxq19991111]

Summary

Fixes #1022.

This PR fixes dead Streamable HTTP sessions accumulating when keep-alive pings fail after a client disconnects.

• Add success and failure callbacks to KeepAliveScheduler, with default no-op handlers so existing scheduler users
  keep the same behavior.
• Track consecutive keep-alive ping failures in HttpServletStreamableServerTransportProvider.
• Evict and close a Streamable HTTP session after 3 consecutive keep-alive ping failures.
• Clear failure counts after successful pings, session deletion, shutdown, and transport send failure.
• Guard failure-count reset and eviction with session identity checks so stale keep-alive results cannot affect a
  replaced session.
• Add tests for scheduler callbacks and Streamable HTTP keep-alive eviction behavior.

Notes

The issue suggested doing the counter/removal directly in KeepAliveScheduler. This PR keeps KeepAliveScheduler
transport-agnostic and exposes success/failure hooks instead. The Streamable HTTP servlet provider owns the session
map, so it also owns eviction.

This PR intentionally does not modify the deprecated HttpServletSseServerTransportProvider.

This PR also does not include the separate async lifecycle / socket cleanup work.

Testing

• mvn -pl mcp-core -Dtest=KeepAliveSchedulerTests,HttpServletStreamableServerTransportProviderTests test
• mvn -pl mcp-core test