[fix] regex: error on a group index that overflows int64 (LET-25)

[vt128]

What

Match.group / start / end / span resolve their group selector through groupIndex, which called starlark.Int.Int64() and discarded the ok flag. For an index too large for int64, Int64() returns 0, so the bounds check passed and the call silently selected group 0 (the whole match) instead of erroring — m.group(1 << 70) returned the full match text.

Fix

Capture ok and reject an overflowing (or out-of-range) index with no such group, comparing as int64 to avoid truncation and formatting the original value (which prints as 0 via %d). groupIndex is the single chokepoint, so this fixes group/start/end/span together — matching CPython, which raises IndexError: no such group.

Test-first

A new section in regex_test.go asserts group/start/end/span all fail with no such group on 1 << 70; it passes for a normal index. Fails before the fix.

Verification

go test -race -count=2 ./..., go vet, gofmt -l clean, Docker golang:1.19 race run green.

Requirement: LET-25

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 0 complexity · 0 duplication

Metric	Results
Complexity	0
Duplication	0

View in Codacy

🟢 Coverage 100.00% diff coverage · +0.02% coverage variation

Metric	Results
Coverage variation	✅ +0.02% coverage variation (-1.00%)
Diff coverage	✅ 100.00% diff coverage

View coverage diff in Codacy

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (433eae0)	7692	7276	94.59%
Head commit (2486917)	7697 (+5)	7282 (+6)	94.61% (+0.02%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#164)	8	8	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.41%. Comparing base (433eae0) to head (2486917).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #164      +/-   ##
==========================================
+ Coverage   93.39%   93.41%   +0.01%     
==========================================
  Files          49       49              
  Lines        6177     6177              
==========================================
+ Hits         5769     5770       +1     
+ Misses        260      258       -2     
- Partials      148      149       +1     

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