test(review): cover review-diff.ts non-positive-budget guard and header count defaults

[jaytbarimbao-collab]

Fixes #5849.

src/review/review-diff.ts had untested branches around its budget/size handling. This adds direct unit tests (no production change) for:

• keepHighSignalHunks non-positive budget guard — budget <= 0 (zero and negative) returns the truncation marker.
• single-hunk head-slice — a patch with no second @@ header (hunks.length <= 1) that exceeds the budget is head-sliced with the truncation marker, not dropped whole.
• buildUnifiedReviewDiff header defaults — a file with missing additions/deletions renders +0/-0, and a missing status defaults to modified; provided counts are used when present.
• below-floor truncation — once the remaining budget falls under the per-file floor, the diff stops with the …diff truncated (N files total) notice.
• patch-less file listing — a binary/too-large file is still listed with its counts.

Branch coverage of the file rises from 66.7% to 77.3%. Test-only; all existing tests still pass (19 total).

[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 74.75%. Comparing base (db74aa3) to head (dde8fc4).
⚠️ Report is 10 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (db74aa3) and HEAD (dde8fc4). Click for more details.

HEAD has 3 uploads less than BASE

Flag	BASE (db74aa3)	HEAD (dde8fc4)
shard-5	1	0
shard-3	1	0
shard-4	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5857       +/-   ##
===========================================
- Coverage   95.11%   74.75%   -20.37%     
===========================================
  Files         586      586               
  Lines       46480    46480               
  Branches    14860    14860               
===========================================
- Hits        44209    34744     -9465     
- Misses       1515     8682     +7167     
- Partials      756     3054     +2298     

Flag	Coverage Δ
shard-1	43.31% <ø> (-0.42%)	⬇️
shard-2	36.19% <ø> (-0.05%)	⬇️
shard-3	?
shard-4	?
shard-5	?
shard-6	44.89% <ø> (+0.35%)	⬆️

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see 282 files with indirect coverage changes

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✅ LoopOver review result - approve/merge recommended

_{Review updated: 2026-07-14 20:19:33 UTC}

1 file · 1 AI reviewer · no blockers · readiness 100/100 · CI green · clean

✅ Suggested Action - Approve/Merge

• safe to merge

Review summary
This is a test-only PR that adds direct unit tests for four previously-uncovered branches in review-diff.ts: the keepHighSignalHunks non-positive-budget guard, the single-hunk head-slice path, buildUnifiedReviewDiff's header defaults for missing additions/deletions/status, and the below-floor truncation notice. I traced each new test against the current source in review-diff.ts and all assertions match actual behavior (budget<=0 returns the marker, hunks.length<=1 head-slices with slice(0,budget), header uses `??0`/`??"modified"` defaults, and the loop breaks with the truncation message when remaining<240). No production code is touched.

Nits — 4 non-blocking

• The below-floor truncation test (test/unit/review-diff.test.ts new describe block) doesn't assert which file triggered the truncation cutoff, just that the message appears — a slightly tighter assertion (checking diff.length or that only one file's header appears) would pin down the exact boundary being exercised.
• The head-slice test only checks `out.startsWith(single.slice(0, 30))` and that it contains the truncation marker, but doesn't assert the exact expected output string, leaving a small gap versus the more precise equality checks used elsewhere in the file (e.g. the exact-budget-fit test).
• Consider adding one boundary test at exactly `budget === 0` vs `budget === -1` collapsing into the same code path is already implied by the two guard tests, so no change needed there — this is just an observation, not an ask.
• The PR could mention the resulting branch-coverage percentage per describe block for clarity in the description, though the aggregate 66.7%→77.3% figure is already given.

Signal	Result	Evidence
Code review	✅ No blockers	1 reviewer
Linked issue	✅ Linked	#5849
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: 75 registered-repo PR(s), 36 merged, 4 issue(s).
Contributor context	✅ Confirmed Gittensor contributor	jaytbarimbao-collab; Gittensor profile; 75 PR(s), 4 issue(s).
Gate result	✅ Passing	No configured blocker found.
Improvement	⚠️ ℹ️ Insufficient signal	risk: clean · value: insufficient-signal — Nothing measurable for the structural-improvement analyzers on this PR (e.g. no code files changed). LLM value judgment: moderate — The PR adds real, source-traced unit tests for genuinely untested branches (non-positive budget guard, single-hunk head-slice, header defaults, floor truncation) in a hot-path diff-building module, raising branch coverage by ~10.6 points with no risk since it's test-only.

Linked issue satisfaction

Addressed
The PR adds direct test cases for both requested gaps — keepHighSignalHunks with budget 0 and negative returning the truncation marker, and buildUnifiedReviewDiff producing +0/-0 for a file missing additions/deletions — matching the issue's exact requirements, plus additional related coverage.

Review context

• Author: jaytbarimbao-collab
• 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: 75 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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