Investigate potential OTel context leak in vector store background tasks

[iamemilio]

Summary

PR #5168 fixed OTel context leaks in the InferenceStore and OpenAIResponsesImpl background workers by introducing create_task_with_detached_otel_context. During review, @jaideepr97 identified three additional asyncio.create_task call sites in openai_vector_store_mixin.py that may have the same bug — background tasks inheriting and permanently retaining the creating request's OTel trace context.

Potentially affected locations

All in src/llama_stack/providers/utils/memory/openai_vector_store_mixin.py:

1. Line ~394 — Resuming a file batch on startup:

   task = asyncio.create_task(self._process_file_batch_async(batch_id, batch_info, remaining_files))

2. Line ~1317 — Starting background processing of a new file batch:

   task = asyncio.create_task(self._process_file_batch_async(batch_id, batch_info))

3. Line ~1327 — Running throttled cleanup of expired file batches:

   asyncio.create_task(self._cleanup_expired_file_batches())

Expected behavior

Each background task should either:

• Start with a detached OTel context (using create_task_with_detached_otel_context), or
• Carry per-item context from the originating request (using capture_otel_context / activate_otel_context)

so that spans are attributed to the correct request trace and don't leak across unrelated requests.

Steps to reproduce

1. Enable OTel tracing (e.g. export to Jaeger)
2. Send concurrent requests that trigger vector store file batch operations
3. Inspect traces in Jaeger — look for inflated trace durations or spans from unrelated requests appearing under a single trace

Additional context

• See PR fix: prevent OTel context leak in fire-and-forget background tasks #5168 for full explanation of the root cause (asyncio.create_task copies all contextvars at creation time)
• The fix utilities (create_task_with_detached_otel_context, capture_otel_context, activate_otel_context) are already available in llama_stack/core/task.py
• These locations may behave differently from the ones fixed in fix: prevent OTel context leak in fire-and-forget background tasks #5168 (e.g. shorter-lived tasks vs. long-lived workers), so testing is needed to confirm whether the leak actually manifests here