Handle rejected handshakes cleanly in the threading server

[aopv]

Summary

The threading server didn't handle a rejected opening handshake explicitly. When process_request or process_response returned an HTTP response, handshake() returned normally while the protocol state wasn't OPEN. The following assertion failed silently—or disappeared under python -O, allowing the handler to run on a closed connection and log "connection handler failed".

Fix

Replace the assertion with an explicit rejection path that closes the socket, joins the receive thread, and returns, mirroring the asyncio implementation.

Tests / Verification

• Guarded test_process_request_returns_response with assertNoLogs("websockets", logging.ERROR).
• Matched the asyncio tests by verifying the exact "opening handshake failed" error and exception for process_request and process_response failures.
• Official coverage environment: 2,258 tests passed, 10 skipped, 100% coverage.
• Ruff formatting and lint checks pass.
• Strict mypy checks pass.
• python -O -m unittest tests.sync.test_server: 50 tests passed.

Refs #1513.

Disclosure: prepared with AI assistance; manually reviewed and verified locally.

[aaugustin]

Can you change the commit message to something shorter, e.g.:

Don't log an error when process_request returns a response.

Refs #1513.

We don't need a novella :-)

[aaugustin]

I don't add changelog entries for small fixes like this one (e.g. cdeb882, which fixed the same problem in the asyncio implementation, didn't include one).

[aaugustin]

Remove this line. We're in a branch "state is not OPEN"; it's clear that it's closing or closed.

[aaugustin]

If you look at cdeb882, you'll see that it added assertNoLogs assertions to a bunch of tests in the asyncio implementation.

Likely, the same tests should have assertNoLogs in the threading implementation. Can you check and add where appropriate please?

[aaugustin]

The failure in GitHub Actions looks spurious :-/

[aaugustin]

Thank you, this looks valid. I'd like a few changes; you're welcome to update the PR if you'd like; or I can make them myself and take it from there.

[aaugustin]

Looks perfect. Thank you very much!