fix regex remove duplicates

[hackwaly]

Closes #4074

The previous implementation re-create seen every iteration, which is wrong behavior.

[coveralls]

Coverage Report for CI Build 6128

Coverage decreased (-0.005%) to 90.715%

Details

• Coverage decreased (-0.005%) from the base build.
• Patch coverage: 2 of 2 lines across 1 file are fully covered (100%).
• 1 coverage regression across 1 file.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

1 previously-covered line in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
string/internal/regex_engine/execute.mbt	1	92.0%

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Coverage Stats

Relevant Lines:	17706
Covered Lines:	16062
Line Coverage:	90.72%
Coverage Strength:	324026.95 hits per line

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💛 - Coveralls

[Copilot]

Pull request overview

This PR addresses regex execution blowups caused by inadequate thread deduplication in variable-length repetition bodies (as described in #4074), preventing exponential growth in the thread set and related OOM behavior.

Changes:

• Add a regression test that exercises variable-length repetition patterns over a long subject and ensures matching remains feasible.
• Fix ThreadSet::remove_duplicates to share a single seen set across nested Seq thread trees, enabling global deduplication across wrapper boundaries.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
string/regex_test.mbt	Adds a regression test covering variable-length repetition cases that previously led to exponential thread growth/OOM.
internal/regex_engine/automata/thread_set.mbt	Updates duplicate-thread elimination to reuse a single seen set across nested thread trees, preventing exponential growth.

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