fix(runtime): separate dash-prefixed Grep patterns with -- in sandbox worker

[yunaremaia]

Summary

• The sandboxed filesystem worker appended the Grep pattern as a bare positional, so ripgrep parsed a dash-prefixed pattern as flags. A leading flag exits 1, and this worker maps exit 1 to an empty match set - so the model was told a present string was absent, with no error.
• Adds the -- argv separator the host-local workspace executor already pins (workspace-executor.ts, from feat(windows): add brokered AppContainer sandbox support #2961), plus a regression test asserting the separator position and a successful -webkit-box search.

Test plan

• New regression test fails without the fix (asserts -- precedes the pattern in the spawned argv) and passes with it
• Reproduced against real ripgrep 15.1.0: rg -n --no-heading --max-count=5 -webkit-box style.css exits 1; with -- it matches
• Full runtime suite locally: 3059/3068 pass; the 2 failures are pre-existing root-user permission tests (chmod-restricted files are readable by root, CI runs non-root) - untouched by this diff

Fixes #3733

[M4n5ter]

Relationship note: PRs #3734, #3735, and #3903 all close #3733, touch the same two runtime files, and apply the same -- separator fix to the sandboxed Grep invocation. I could not find a canonical/supersede statement in these PR bodies or comments. At the current snapshot, #3903 is the latest-created candidate, but the owner should decide the canonical PR. Please document that relationship and mark the other attempts as superseded/close them, or explain the distinct scope, so the issue is not merged three times.

This comment records the relationship only; it is not a merge decision.

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Automated review note posted by @未开智选手. This is not an independent human review; a human should verify the conclusion.

[M4n5ter]

Thank you for working on this fix. I compared this implementation with #3735 and #3903. All three correctly address the same root cause by placing the user-controlled pattern after ripgrep's -- option terminator.

#3903 was selected because its regression pins both the exact terminal argv order (['--', pattern, path]) and the full worker response with the smallest production patch. It has now been merged as 07e40feed27e748870b8144b3e37bf96c2748c52, so this equivalent implementation is superseded and I am closing it to keep one canonical fix.

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