Wake a WebSocket read on a pong timeout on every platform

When max_missed_pongs ran out, the heartbeat thread shut down the read
side of the socket to end a read() blocked on the silent peer. On
Windows that does not wake the wait, so read() stayed blocked until its
read timeout and then returned Timeout. The heartbeat test never ran on
Windows, so CI did not catch it.

The heartbeat now sets a flag that the stream's read interrupt checks,
the mechanism SSEClient::stop() uses, and WebSocketSSLStream takes the
interrupt too. It is set only when a pong timeout can happen, so other
connections wait as before. read() reports a timeout on a connection
that has closed as Fail, since Timeout means it is still usable.

Build and run the heartbeat test in the Windows jobs, and add a TLS
pong timeout test.

.github/workflows/test.yaml

@@ -623,6 +623,10 @@ jobs:
         }
         if ($failed) { exit 1 }
         Write-Host "All shards passed."
+    - name: Run WebSocket heartbeat test ${{ matrix.config.name }}
+      if: ${{ matrix.config.run_tests }}
+      working-directory: build/test
+      run: ./Release/httplib-heartbeat-test.exe --gtest_color=yes
 
     env:
       VCPKG_ROOT: "C:/vcpkg"