fix: prevent head-of-line blocking in RedisWorker fetch_task() (#7900)

[dkliban]

Summary

When the queue head is dominated by tasks needing the same blocked exclusive resource (e.g., 17,000+ general_create tasks for the same Python repository), fetch_task() scanned from position 0 on every iteration, discarding knowledge of blocked resources and starving tasks with free resources for hours.

Fix: Track resources reported as blocked by acquire_locks and exclude them from subsequent DB queries using PostgreSQL's array overlap operator (&&), leveraging the existing GIN index (pulp_task_resources_index) on reserved_resources_record.

Key changes

• Accumulate blocked_resources set across fetch_task() iterations
• Exclude tasks with blocked resources at DB level via reserved_resources_record__overlap
• Preserve taken_exclusive/taken_shared FIFO knowledge across iterations
• Use fixed-size batches instead of exponential doubling from position 0

Properties

• FIFO fairness within same resource preserved -- tasks for the same resource still execute in creation order
• No changes to Redis lock mechanism -- acquire_locks Lua script unchanged
• No changes to handle_tasks() interface -- fetch_task() still returns Task or None
• blocked_resources is local to each fetch_task() call -- starts fresh after each task completes
• Conservative resource exclusion -- only raw resource names added (not shared: prefixed)

Test plan

• Unit test: 200 tasks on 10 blocked resources + 1 free task -- verifies acquire_locks called <= 21 times (was 51)
• Functional test: 30 blocked tasks + 1 free-resource task -- verifies free task completes within 30 seconds
• Test discrimination verified (fails on original code, passes on fix)
• Regression analysis: 0 regressions
• Adversarial review: 0 findings

Closes: #7900

[dkliban]

@gerrod3 I addressed your feedback. PLease re-review. Thank you!

[gerrod3]

Looks good, thanks!

[patchback]

Backport to 3.116: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.116/ff9930d1996a4da8f54f7dd97d88aca56479b332/pr-7987

Backported as #7989

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