fix(tool): preserve colon paths in code_search

[RerankerGuo]

Description

code_search parsed git grep output by splitting each line on :. A valid
path such as foo:bar.go therefore produced:

foo:bar.go:2:// needle

The parser treated bar.go as the line number, failed integer conversion, and
silently dropped the match.

This change requests Git's NUL-delimited output and parses records as:

path\0line\0content\n

Commit-mode results keep their existing ref:path prefix handling, while the
public output format remains unchanged. A real-repository regression test
covers both a colon-containing path and an ordinary path in workspace and
commit modes.

The existing per-file result cap and truncation-reporting semantics are
intentionally unchanged.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactoring (no functional changes)
• Documentation update
• CI / Build / Tooling

How Has This Been Tested?

• make check
• make test
• make build
• make coverage - 90.2%, meeting the 90% threshold
• Real temporary Git repositories in workspace and commit modes

Before the fix, the regression test returned only plain.go; the
foo:bar.go match disappeared. After the fix, both files retain the expected
line number and content in both review modes. The focused test and the full
internal/tool package pass with the race detector.

Self-review used OCR Delegation Mode with the host agent:
2/2 changed files reviewed, 0 skipped, 0 blocking findings.

Checklist

• My code follows the project's coding style (go fmt, go vet)
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or my feature works
• New and existing unit tests pass locally with my changes
• Documentation is not required for this internal output parser fix
• I have signed the CLA

Related Issues

No existing issue found. All open issues, pull request text, and open pull
request changed files were checked before implementation; no competing
code_search path-parser change was found.

[github-actions]

✅ OpenCodeReview: Review complete: 0 finding(s) across 1 selected item(s).