Complete frontend_path validation for Win32-trimmed and empty segments

[masenf]

The frontend-path guard added in #7044 still accepts directory names that Win32 trims and repeated separators. Reject trailing dots/spaces and empty internal segments before building or relocating the frontend, while retaining root paths, one optional trailing slash, interior spaces, and ordinary dotted names.

Addresses prerelease finding 014.

Validation: seven new unsafe-path cases failed before the fix; all 142 config tests pass afterward. Ruff lint and formatting pass. Reviewed boundary cases for empty/root prefixes and legitimate names.

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge; the validation consistently rejects the targeted unsafe forms without disrupting the explicitly retained path forms.

Summary

• Rejects path segments ending in dots or spaces that Win32 trims.
• Rejects empty internal segments introduced by repeated separators.
• Preserves empty and root paths, one optional boundary slash, interior spaces, and ordinary dotted names.
• Adds regression coverage and a downstream-facing bugfix fragment.

_{Reviews (1) · Last reviewed commit: "fix(config): reject trimmed and empty fr..."}

[codspeed-hq]

Merging this PR will not alter performance

✅ 40 untouched benchmarks
⏩ 8 skipped benchmarks¹

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_{Comparing codex/prerelease-014-path-validation (4404c19) with main (074a818)}

Footnotes

1. 8 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩