Suppress spurious "Failed to detach context" log in OTel interceptor

[brianstrauch]

What

Flaky test observed here: https://github.com/temporalio/sdk-python/actions/runs/27432588766/job/81086412642?pr=1378

TracingWorkflowInboundInterceptor attaches an OTel context at the start of execute_workflow/handle_query and detaches it in the finally. The workflow event loop runs portions of the workflow inside contextvars.copy_context().run(...) (e.g. wait_condition), so that finally can execute in a copied contextvars.Context. A copy preserves the OTel context value — so the existing context is get_current() guard passes — but invalidates the attach token, so opentelemetry.context.detach fails internally and logs "Failed to detach context".

Because LogCapturer attaches a handler to the process-global opentelemetry.context logger, that stray ERROR log bled into test_opentelemetry_safe_detach whenever a tracing workflow tore down during its capture window (same xdist worker process), making the test flaky. In production it's recurring ERROR-level noise on affected workflow completions.

Fix

Route the best-effort detach through a small _safe_detach helper that drops only the "Failed to detach context" record via a scoped logging.Filter.

It still calls opentelemetry.context.detach, so attach/detach call counts stay balanced — the leak invariant enforced by test_opentelemetry_context_restored_after_activity. (Alternatives that avoid the public detach — restoring via attach(previous), or resetting the runtime contextvar directly — both break that balance test, so suppressing at the logger is the only approach that satisfies balanced calls + no spurious log + no test weakening.)

Caveat

During a detach the filter is attached to the process-global OTel context logger, so it would also drop an unrelated "Failed to detach context" emitted by other code in that brief window. Given the alternatives break a correctness invariant, this is the right trade-off.

Testing

• tests/contrib/opentelemetry/ and tests/contrib/openai_agents/test_openai_tracing.py pass together (27/27), including the attach/detach balance test.
• Verified directly that the cross-context detach logs "Failed to detach context" via raw detach but produces no record through _safe_detach.
• Lint clean (ruff, format, pyright, mypy, basedpyright, pydocstyle).