fix(search): zero-match steering probe works on the grep fallback

[webtecnica]

Summary

The search_files zero-match steering probe was a silent no-op on hosts without ripgrep installed. _zero_match_probe in tools/file_operations.py returned None immediately when rg was absent (if not self._has_command('rg'): return None), so all three zero-match hints ("case-insensitive matches — casing may be wrong" / "literal matches — metacharacters need escaping" / "match only in hidden or gitignored files") were lost even though the grep fallback search itself worked.

Root Cause

The probe feature added in 5797b5028 was implemented only for the rg engine path; the grep fallback path bailed out before running any probe variants.

Change

• tools/file_operations.py: _zero_match_probe is now a dispatcher — rg → _zero_match_probe_rg (original body, byte-identical, no regression), otherwise → new _zero_match_probe_grep that mirrors the three probe variants using the existing grep-fallback helpers (count-only output, head -50, 2>/dev/null, 30s timeout):
  • case-insensitive: grep -r -i -c -H --exclude-dir='.*' [--include glob] … → "case-insensitive match(es) … casing may be wrong"
  • hidden-only: re-run without --exclude-dir → "N match(es) in N hidden file(s) — hidden files are excluded by default"
  • literal: grep -r -F -c -H (only when the pattern has metacharacters) → "literal match(es) … need escaping"
• tests/tools/test_search_zero_match_and_multipath.py: new coverage for the no-rg path (via _has_command patch) plus the existing rg path.

Verification

• tests/tools/test_search_zero_match_and_multipath.py: 20 passed.

Closes #79512