test: fix flaky http2-dispatcher WebSocket upgrade tests

[mcollina]

The CONNECT/WebSocket upgrade server handlers used stream.respond()
with the default endStream: true, immediately closing the tunnel from
the server side. This caused ECONNRESET because the HTTP/2 session
would tear down the TCP connection before queued requests could
complete or before test assertions could verify socket state.

Changes:

• Dispatcher#Upgrade: respond with { endStream: false } to keep the
  CONNECT tunnel open, move socket error handler before assertions
• Dispatcher#Upgrade resumes queued requests: same endStream fix,
  add server-side end handler to close tunnel when client ends,
  move error handler before assertions, simplify try-finally cleanup

Fixes: #5195

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.24%. Comparing base (314ba6a) to head (f617556).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5304      +/-   ##
==========================================
+ Coverage   93.23%   93.24%   +0.01%     
==========================================
  Files         110      110              
  Lines       36575    36575              
==========================================
+ Hits        34099    34106       +7     
+ Misses       2476     2469       -7     

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