feat: OTel: Task Processor

[khvn26]

Contributes to #184.

• Serialise W3C trace context (traceparent, baggage) into a nullable trace_context JSONField on tasks at enqueue time via propagate.inject() in TaskHandler.delay()
• Extract and activate context as a parent span in _run_task(), linking task execution spans back to the originating HTTP request trace
• Derive default service.name from entrypoint: flagsmith-task-processor when running the task processor, flagsmith-api otherwise (OTEL_SERVICE_NAME overrides both)
• Add opentelemetry-api to task-processor extras, TraceContext type alias, and span attributes matching Prometheus metric labels for cross-signal correlation

How to review

Review complexity: 2/5

Start with the model layer (models.py, types.py, migration) — a new trace_context nullable JSONField. Then follow the write path in decorators.py. Next, the read path in processor.py. Finally, main.py has one conditional for the default service.name based on sys.argv.

Tests mirror the above order: model, decorator, processor, main. The processor tests use static traceparent strings with known hex IDs for parametrisation. span_exporter fixture was promoted from integration test to root conftest, and RunTasksFixture protocol was updated with a default num_tasks.

Test plan

• Run Flagsmith API + task processor against a local collector (e.g. SigNoz)
• Trigger a request that enqueues a task (e.g. flag update that triggers environment rebuild)
• In the trace viewer, verify the task processor span appears as a child of the HTTP request span with matching trace ID
• Confirm span attributes (task_identifier, task_type, result) are present
• Confirm service.name shows flagsmith-task-processor for task processor spans and flagsmith-api for API spans
• Verify structlog events emitted during task execution carry the correct trace_id / span_id

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.05%. Comparing base (1256e87) to head (dab9484).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #197      +/-   ##
==========================================
+ Coverage   96.95%   97.05%   +0.09%     
==========================================
  Files         101      102       +1     
  Lines        4008     4140     +132     
==========================================
+ Hits         3886     4018     +132     
  Misses        122      122              

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[emyller]

Approving because it looks great, my comments are not blocking.

There is one coverage failure I'm suspicious about, but other tests add enough confidence.

[emyller]

👍👍👍