fix(signals): flag the .at(-1)/.at(0) idiom in boundary-test-generation

[lourincedaging0-commits]

Summary

ARRAY_INDEX_BOUNDS_PATTERN in src/signals/boundary-test-generation.ts (the array/index-bounds detector for the #1972 boundary-safe test-generation signal) matched bracket-index forms (arr[arr.length - 1], arr[-1]) and .length comparisons, but not the modern Array.prototype.at() idiom (arr.at(-1), arr.at(0)) — the standard JS/TS way to express exactly the last-element/off-by-one access this detector exists to flag. Verified: ARRAY_INDEX_BOUNDS_PATTERN.test("items.at(-1)") returned false, so that access silently evaded the signal.

Fix

Add a \.at\(\s*-?\d+\s*\) alternative. Kept narrow per the module's own design principle ("deliberately SMALL and PRECISE — false positives are worse than a narrow true-positive set", #1972): only a numeric literal argument (optionally negative) matches. A bare identifier like .at(idx) carries none of the specific -1/0 boundary signal and matching it would reintroduce false-positive noise. Single additive regex alternative — no other pattern, BOUNDARY_PATTERNS, MAX_TOUCHES, or added-line-only scanning touched.

Tests

Extends test/unit/boundary-test-generation.test.ts: array_index_bounds now fires for items.at(-1), items.at(0), rows.at(-2); a negative test confirms .at(idx) (non-literal) does not trigger. All existing bracket-index / .length tests pass unmodified. 100% line coverage on the touched module locally.

Closes #5843

[superagent-security]

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.15%. Comparing base (f56edd2) to head (c86d489).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6006   +/-   ##
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  Coverage   95.15%   95.15%           
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  Files         589      589           
  Lines       46827    46827           
  Branches    14959    14959           
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  Hits        44560    44560           
  Misses       1511     1511           
  Partials      756      756           

Flag	Coverage Δ
shard-1	43.48% <100.00%> (-0.42%)	⬇️
shard-2	35.61% <100.00%> (-0.83%)	⬇️
shard-3	32.80% <100.00%> (+0.81%)	⬆️
shard-4	32.01% <100.00%> (+0.05%)	⬆️
shard-5	32.24% <100.00%> (-0.22%)	⬇️
shard-6	44.91% <100.00%> (+0.18%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/signals/boundary-test-generation.ts	97.14% <100.00%> (ø)

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✅ LoopOver review result - approve/merge recommended

_{Review updated: 2026-07-15 06:51:25 UTC}

2 files · 1 AI reviewer · no blockers · readiness 98/100 · CI green · clean

✅ Suggested Action - Approve/Merge

• safe to merge

Review summary
Adds a single regex alternative (`\.at\(\s*-?\d+\s*\)`) to ARRAY_INDEX_BOUNDS_PATTERN in src/signals/boundary-test-generation.ts so the `.at(-1)`/`.at(0)` idiom is now flagged, matching the existing narrow-scope design (numeric literal only, no bare identifiers). The added tests directly exercise the new alternative (positive cases for `.at(-1)`, `.at(0)`, `.at(-2)`, and a negative case for `.at(idx)`) and correctly validate the regex boundary. This is a minimal, well-targeted, low-risk change that closes the stated gap without touching any other pattern or scanning logic.

Nits — 3 non-blocking

• The pattern won't match `.at(+1)` with an explicit plus sign, though this is an edge case with no real-world prevalence and consistent with the module's narrow-by-design philosophy.
• Consider a brief comment noting that `\d+` intentionally excludes decimal/float arguments like `.at(1.5)`, though such usage would be unusual for array indexing anyway.
• No functional changes suggested — the diff is narrow, tested, and consistent with the module's existing conventions.

Signal	Result	Evidence
Code review	✅ No blockers	1 reviewer
Linked issue	✅ Linked	#5843
Related work	✅ No active overlap found	No same-issue or scoped active PR overlap found.
Change scope	✅ 20/20	Low review scope from cached public metadata (1 linked issue).
Validation posture	✅ 25/25	PR body includes validation/test evidence.
Contributor workload	✅ 10/10	Author activity: 65 registered-repo PR(s), 26 merged, 4 issue(s).
Contributor context	✅ Confirmed Gittensor contributor	lourincedaging0-commits; Gittensor profile; 65 PR(s), 4 issue(s).
Gate result	✅ Passing	No configured blocker found.
Improvement	✅ Minor	risk: clean · value: minor · LLM: minor

Linked issue satisfaction

Addressed
The diff adds the required \.at\(\s*-?\d+\s*\) alternative to ARRAY_INDEX_BOUNDS_PATTERN, leaving other patterns/BOUNDARY_PATTERNS untouched, and adds tests covering .at(-1), .at(0), .at(-2) positive matches plus a .at(idx) negative test as specified.

Review context

• Author: lourincedaging0-commits
• Role context: outside_contributor
• Public audience mode: oss maintainer
• Lane context: Repository is configured for direct PR review.
• Public profile languages: not available
• Official Gittensor activity: 65 PR(s), 4 issue(s).
• PR-specific overlap: none found.

Contributor next steps

• Keep the PR focused and include validation evidence before maintainer review.

Signal definitions

• Related work = same linked issue, overlapping active PRs, or title/path similarity.
• Change scope = cached public metadata such as size labels, draft state, and review-burden hints.
• Validation posture = whether the PR provides enough public validation/test evidence for maintainer review.
• Contributor workload = public contributor activity and cleanup pressure, not a repo-wide quality failure.
• Contributor context = public GitHub/Gittensor identity context; non-Gittensor status is not a blocker.

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_{🟩 Safe / merged · 🟦 Advisory · 🟨 Held for review · 🟥 Blocked / closed}

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LoopOver approves — the gate is satisfied and CI is green.