fix: route HttpProxySink timeouts through spool fallback

[atimothee]

Summary

HttpProxySink._post() only caught HTTPError and URLError, so a TimeoutError (for example a socket-level connect timeout, or any other low-level OSError such as ConnectionRefusedError) bypassed the configured spool_path fallback and surfaced the raw exception to callers instead of the sink's RuntimeError wrapper.

This change catches TimeoutError and OSError alongside the existing URL errors so the spool write and the sink's RuntimeError wrapper are exercised consistently for every delivery failure, restoring the configured fallback behavior. A short comment documents why the wider exception list is intentional.

Test plan

• Added a parametrized regression test (tests/sandbox/test_session_sinks.py::test_http_proxy_sink_writes_spool_on_timeout_or_oserror) that patches urllib.request.urlopen in the sinks module to raise TimeoutError and OSError in turn, asserts the sink raises RuntimeError with the expected message, and asserts the spool file contains the serialized event line.
• Ran uv run pytest tests/sandbox/test_session_sinks.py -v -k test_http_proxy_sink_writes_spool_on_timeout_or_oserror (both parametrized cases pass).
• Ran uv run pytest tests/sandbox/ -q — 806 passed, 19 skipped (no regressions).
• Ran make format and uv run ruff check/uv run mypy/uv run pyright against the changed files — all clean.

Issue number

Closes #3641

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass

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