Fix strict_cert proxy bypass and loopback SO_REUSEADDR on Windows

[Flerpharos]

Fixes #327

Two low-severity networking fixes in src/labapi/client.py:

1. Proxy TLS bypass: _313HTTPAdapter only overrode init_poolmanager, so the relaxed SSL context (strict_cert=False, which drops VERIFY_X509_STRICT) was never applied to requests routed through an HTTPS proxy — proxy_manager_for built its pool manager with default strict verification, silently ignoring the setting. Fixed by storing the built ssl_context on the adapter and overriding proxy_manager_for to pass the same context through to the parent implementation.

2. Loopback callback SO_REUSEADDR on Windows: the auth callback TCPServer unconditionally set allow_reuse_address = True. On POSIX this is the benign TIME_WAIT-reuse behavior. On Windows, SO_REUSEADDR instead permits another local process to bind the same 127.0.0.1:<port> while the listener is up, letting it race for the auth_code/email callback. Fixed with a platform conditional: on Windows, allow_reuse_address stays False and a server_bind override sets SO_EXCLUSIVEADDRUSE instead; POSIX behavior is unchanged.

Testing

• Added test_strict_cert_false_relaxes_ssl_for_direct_and_proxied_requests, which asserts the adapter stores ssl_context and that proxy_manager_for injects the same context into its pool manager.
• Added test_collect_auth_response_blocks_concurrent_bind_on_windows (skipped off Windows), which binds the loopback listener via client.collect_auth_response() and asserts a second socket (even with SO_REUSEADDR set) cannot bind the same port. Verified this reproduces the pre-fix vulnerability directly at the socket level (two SO_REUSEADDR sockets can share a port on Windows) and that the fix blocks it (PermissionError/WinError 10013).
• Ran the full suite (uv run pytest --ignore=tests/test_integration.py) five times in a row on Windows to check for rebind flakiness from disabling SO_REUSEADDR: consistently 432 passed, 1 skipped, no flakes.
• uv run ruff check . and uv run ruff format --check . pass; pre-commit's pyright hook passes on the changed files.

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