Add tracking for Celery canvas primitives (map/starmap/chunks)

[snopoke]

Summary

• Add tracking for tasks spawned via Celery canvas primitives (map, starmap, chunks)
• Canvas primitives now create TaskBadger tasks using the inner task's name with metadata about canvas_type and item_count
• Add new tests for test_task_map, test_task_starmap, and test_task_chunks

Background

Previously, tasks using canvas primitives like task.chunks(items, n).group() or task.map(items) were not tracked because these execute as built-in celery.map/celery.starmap tasks, which were filtered out by the before_task_publish signal handler.

Implementation

Added _maybe_create_task function called during task_prerun that:

• Creates TaskBadger tasks for canvas primitives using the inner task's name
• Includes metadata: {"canvas_type": "celery.starmap", "item_count": 3}
• Carefully orders checks to avoid thread-local Badger state pollution
• Respects existing tracking intent via taskbadger_track header

Test plan

• All existing celery tests pass
• New tests for map, starmap, and chunks canvas primitives
• Manual verification with eager mode

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