file-search: ignore parent gitignore rules

[demfabris]

Fixes a file-search behavior where ignore rules from parent directories above the search root could incorrectly exclude results.

• Always set WalkBuilder::parents(false) so ignore rules don’t leak from above the search root
• Adds regression tests verifying parent .gitignore does not apply, while root .gitignore still does

[github-actions]

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[demfabris]

I have read the CLA Document and I hereby sign the CLA

[etraut-openai]

Thanks for the contribution. Is there an active bug report associated with this PR? We ask that all PRs have bug reports linked to them. It helps us track and prioritize issues, especially in cases where a PR is rejected but the bug remains.

In this particular case, I'm not sure the proposed change is desirable. I'd like to better understand what problem this is solving, so a bug report would also help there.