[PR] Fix automatic pong close race [tomcat]

moritzfl opened a new pull request, #1041:
URL: https://github.com/apache/tomcat/pull/1041

   ## Fix automatic Pong/session-close race
   
   A WebSocket session may begin closing after `WsFrameBase` checks `WsSession.isOpen()` but before it calls `sendPong()`. In that interval, `sendPong()` throws `IllegalStateException` on the inbound frame-processing thread.
   
   Suppress this exception only when closing has begun; propagate it otherwise. Add regression coverage for both outcomes.
   
   ### Backport applicability
   
   The same non-atomic `isOpen()` / `sendPong()` sequence exists in all supported release branches: `11.0.x`, `10.1.x`, and `9.0.x`. These should be considered for backport after merge to `main`. We would need a fix for `11.0.x` at the company I work for.
   
   ### Testing
   
   `ant test` with JDK 21.0.9 ran for 57m57s on my Windows machine but failed due to broad NIO2 test discovery failures (`No tests found`) affecting unrelated tests. The added `TestWsFrame` coverage passed under NIO: 4 tests, 0 failures.


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