fix(sessions): keep a matched rg path even at the repo root

[drakeo338]

Summary

• Fixes A Codex rg search that finds a file at the repository root binds nothing, because the result must contain a slash #2623.

The Codex adapter's _rewrite_search_output dropped every root-level file (app.py, setup.py) from rg search results, because it kept a candidate only when it contained a path separator, even for lines _RG_MATCH_RE had already matched as path:line: content. That left search_codebase results empty for root-level matches, so _bind_first_file had nothing to bind the search call's path to. The fix keeps a regex-matched path unconditionally and loosens the bare-line fallback to admit a root-level filename (no separator but a "." after the first character) while still dropping stray prose lines.

Related Issues

Fixes #2623.

Test Plan

• Tests pass (pytest) — ran uv run pytest tests/unit/sessions/test_codex_adapter.py -k codex -q on this branch's HEAD: 22 passed, 0 failed. Did not run the full tests/providers/ tests/unit/ suite.
• Lint passes (ruff check .) — ran ruff check on the two changed files only, not the whole repo: no findings.
• Web build passes (npm run build) (if frontend changes) — no frontend changes, not run.

Checklist

• My code follows the project's code style — ruff check on the touched files is clean (see Test Plan).
• I have added tests for new functionality — three new tests added to tests/unit/sessions/test_codex_adapter.py.
• All existing tests still pass — only ran the scoped test file above, not the full suite, so leaving this unticked.
• I have updated documentation if needed — no doc changes needed for this bugfix.

[repowise-bot]

Reviewed the whole diff: nothing to fix.

⚠️ Health of changed files: 6.2 → 6.0 (-0.1)

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[drakeo338]

Review please?

[RaghavChamadiya]

Thanks @drakeo338, and sorry this sat a couple of days after your ping. Trusting the :\d+: match as proof of a path and only applying the bare-path heuristic to unmatched lines is the right split, and the prose-is-still-dropped test keeps the heuristic honest. Not a blocker: an extensionless root file like Makefile is still dropped, which is fine to leave for another day.