Python(fix): pytest exit instead of raise on connection fail

[alexluck-sift]

Summary

On a failed connectivity ping in online mode, the pytest plugin now aborts the session with pytest.exit instead of raising pytest.UsageError.

Previously the ping ran during the first gated test's setup, so a session-scoped fixture failure was cached and re-raised once per test. A connection failure produced one error block per collected test, with the gRPC error truncated in the summary lines.

pytest.exit stops the session on the spot and prints the message once, before any test body runs, with the full error untruncated. The message text and exit code (4) are unchanged.

Notes

The check stays in the report_context fixture rather than moving to a collection hook, so the user-overridable sift_client and client_has_connection fixtures still resolve as documented.

[github-actions]

Python docs preview: https://sift-stack.github.io/sift/python/pr-606/

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