test(tracing): mock urlopen as a context manager in flush_pending test

[Yeriahz]

What does this PR do?

Fixes a broken mock in test_pending_threads_self_evict_when_post_returns. Test-only change; no production code touched.

Problem

The test patches urlopen with side_effect=fast_post, where fast_post returns None. The production code at src/nooa/tracing/_litellm_journal.py:218 uses the result as a context manager:

with urllib.request.urlopen(req, timeout=timeout) as resp:

with None: raises TypeError: 'NoneType' object does not support the context manager protocol, so every POST fails, retries three times, and takes ~4s. The test waits at most 5s for _PENDING_THREADS to drain, which makes it flaky on slower machines.

More importantly, the test never exercises what its name and docstring describe. The POST never returns; it errors and gives up, so the eviction being verified happens on the failure path rather than the success path. That holds regardless of hardware.

Fix

Return a MagicMock with __enter__/__exit__ defined, matching the existing _ok_response() helper in tests/tracing/test_http_exporter_drops.py:51.

Related issues

None.

Validation

uv run pytest tests/tracing/test_flush_pending.py::test_pending_threads_self_evict_when_post_returns

Before: fails 4 of 5 runs, ~5s each. After: passes 5 of 5, ~0.07s each.

uv run pytest tests/tracing/ -q                      # 181 passed
uv run ruff format --check / ruff check              # clean
uv run pytest -m "not integration and not stress"    # 6334 passed (baseline 6332)

Two unrelated timing tests fail intermittently in my environment (a 4-vCPU VM); the set differs run to run and includes tests in modules this change doesn't touch.

Checklist

• Code follows the project style (uv run ruff check . and uv run ruff format --check . pass)
• Tests added/updated and passing (uv run pytest)
• Docs updated if behavior or public APIs changed — no behavior change
• New source files carry an SPDX license header — no new files

[alessiodevoto]

Hi @Yeriahz thanks for catching this! LGTM, merging!