queue: dedupe backfill mapWithConcurrency helper

[aryansk]

Summary

• reuse the canonical queue mapWithConcurrency helper from backfill
• pass stable item indexes through the shared helper
• add focused concurrency/index regression coverage

Fixes #10289

Verification

• focused prequeue concurrency/index regression harness: passed
• TypeScript compile of the changed canonical helper: passed
• relay-compatible unified patch apply/whitespace check: passed
• full LoopOver Vitest suite could not be run in the execution container because the repository dependency toolchain is unavailable here; draft CI should confirm it

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

❌ 2 Tests Failed:

Tests completed	Failed	Passed	Skipped
12960	2	12958	16

View the top 2 failed test(s) by shortest run time

test/unit/routes-gate-outcome-breakdown.test.ts > GET /v1/app/maintainer-dashboard gateOutcomeBreakdown (#2203) > excludes non-terminal and non-gate audit rows from gateOutcomeBreakdown totals

Stack Traces | 0.142s run time

AssertionError: expected { windowDays: 30, …(5) } to match object { total: 1, counts: { …(3) } }
(7 matching properties omitted from actual)

- Expected
+ Received

  {
    "counts": {
      "autoClosed": 0,
-     "autoMerged": 1,
+     "autoMerged": 0,
      "held": 0,
    },
-   "total": 1,
+   "total": 0,
  }

 ❯ test/unit/routes-gate-outcome-breakdown.test.ts:147:56

test/unit/routes-gate-outcome-breakdown.test.ts > GET /v1/app/maintainer-dashboard gateOutcomeBreakdown (#2203) > surfaces repo-scoped gate-outcome counts on qualityDashboard for an owner session

Stack Traces | 0.302s run time

AssertionError: expected { windowDays: 30, …(5) } to match object { windowDays: 30, total: 3, …(2) }
(2 matching properties omitted from actual)

- Expected
+ Received

  {
    "counts": {
-     "autoClosed": 1,
-     "autoMerged": 1,
-     "held": 1,
+     "autoClosed": 0,
+     "autoMerged": 0,
+     "held": 0,
    },
    "rates": {
-     "autoClosed": 33.3,
-     "autoMerged": 33.3,
-     "held": 33.3,
+     "autoClosed": null,
+     "autoMerged": null,
+     "held": null,
    },
-   "total": 3,
+   "total": 0,
    "windowDays": 30,
  }

 ❯ test/unit/routes-gate-outcome-breakdown.test.ts:79:56

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