[pengpengyi92]

Summary

After the first usage_scope registers the process-global collector, unrelated SDK traces are retained indefinitely. A span that finishes after its scope exits can also recreate an already-evicted bucket.

Collect spans only for explicitly active scope buckets. Opening, appending, and removing a bucket use the same lock; unrelated and late callbacks cannot recreate it. Preserve token/time aggregation and additive SDK processor registration. Add seven offline regression cases using an isolated real SDK provider, covering unrelated/late spans, nested and concurrent scopes, child tasks, exceptions, and cancellation.

Related Issue

Follow-up to #130. No existing issue was found for this collector-retention defect. This does not implement the batch accounting or budget work in #127 / #132.

Verification

• bash scripts/verify.sh on Linux, Python 3.12.14, frozen-lock dependencies including Agents SDK 0.18.3: unchanged baseline 485 passed, 86.45% coverage; candidate 492 passed, 86.50% coverage. All five harness steps pass. The pre-push hook also verified the exact committed tree in a fresh directory with networking disconnected.
• python -B -m pytest tests/utils/test_usage.py --no-cov -p no:cacheprovider -q on Windows, Python 3.12.13 / SDK 0.18.3: 20 passed. The two retention regressions fail against the original implementation before the fix.
• Disposable real-SDK probe: 1,000 unrelated retained spans → 0, one late retained span → 0; completed-scope aggregation remains 15 tokens / one request.
• pre-commit run --files quantmind/utils/usage.py tests/utils/test_usage.py, commit hooks, and git diff --check: pass.
• No live-network smoke test applies: the changed helper observes local SDK tracing only. Regression tests use a separate provider with no exporter and make no model calls.
• Platform limitation: the unchanged full Windows baseline has 21 failures / 464 passes in ETL filesystem decoding, SQLite cleanup, and PDF tests. The full candidate Windows suite was not rerun; its focused usage suite passes. Linux full-suite results above are the release evidence, not a claim of Windows-wide compatibility.

An active scope still retains its completed spans until exit. This fixes unowned/closed-scope retention, not a fixed memory limit or an RSS/throughput guarantee.

Suggested labels: type: bug, area: utils.

Checklist

• The title uses English Conventional Commit format: type(scope): summary.
• The related issue or design discussion is linked when applicable.
• bash scripts/verify.sh passes.
• Every applicable live-network component smoke test passes, or this PR states why none applies.
• Public behavior has focused tests, an example, and documentation where applicable.
• The PR is complete, small, and contains no unrelated changes.