fix(auth): let Ctrl+C abort the login callback wait on Windows - #1446

Context

On Windows, ggshield auth login (browser flow) cannot be interrupted with Ctrl+C while it waits for the browser callback. The command stays blocked until the login completes in the browser or the terminal is closed. Linux and macOS abort as expected.

Root cause: the local callback server waits in handle_request() with no timeout, so select() blocks until the redirect arrives. Python runs signal handlers only between bytecodes. On POSIX, SIGINT interrupts poll() with EINTR, so the handler runs at once. On Windows, Ctrl+C is delivered on a separate console thread that only sets a flag, WinSock select() is never woken, and the KeyboardInterrupt (mapped to click.Abort) only fires once the callback arrives. Known CPython limitation, see python/cpython#133223.

What has been done

• Give the callback server a 0.5s timeout (CALLBACK_POLL_INTERVAL_SECONDS). The wait loop already re-enters handle_request() until the callback is processed, so the only change is that a pending interrupt is now raised within that bound.

• Give the request handler the same timeout. A peer that connects but never sends a request line (browser pre-connect, port scan) used to block readline() with no timeout, one level below the server's select(). The stdlib closes such a connection on the read timeout and the wait resumes.

• Two regression tests in tests/unit/verticals/auth/test_oauth.py, both marked enable_socket because unit tests run with --disable-socket:

  • flip the finished flag from another thread and check the wait loop notices;
  • raise SIGINT from a helper thread (this trips Python's flag without waking select(), the same shape as Windows delivery) and check click.Abort is raised within the bound. If the interrupt never fires, a 5s safety net ends the loop so the test fails with DID NOT RAISE instead of hanging, and both timers are cancelled so a stray SIGINT cannot hit a later test;
  • connect a silent peer first, then flip the finished flag, and check the loop still returns promptly.

  Both fail without the timeout and pass with it on Linux. The windows-2022 legs run them with real Windows signal machinery.

• Changelog fragment.

Validation

• uv run pytest --disable-socket tests/unit/verticals/auth tests/unit/cmd/auth: 182 passed.
• Red/green checked: with the previous oauth.py, the two new tests fail (loop still alive after 3s; abort only after the 5s safety connection).
• Probe scripts on Linux: KeyboardInterrupt after 3.00s without the timeout (only once a connection arrived), 0.50s with it.
• Windows VM (Server 2022, Python 3.13.15), real console Ctrl+C with a silent peer connected to the callback port: without the handler timeout the process was still running 10s after Ctrl+C; with it, exit after 0.30s and 0.30s. Plain Ctrl+C: 0.33s.
• Manual, on Windows: run ggshield auth login, do not complete the browser login, press Ctrl+C. The command should exit within about half a second. On Linux/macOS it still aborts immediately.

PR check list

• As much as possible, the changes include tests (unit and/or functional)
• If the changes affect the end user (new feature, behavior change, bug fix) then the PR has a changelog entry (see doc/dev/getting-started.md). If the changes do not affect the end user, then the skip-changelog label has been added to the PR.