[codex] Honor file_pattern in BM25 graph search

[pcristin]

Summary

• Apply file_pattern filtering to the BM25 search_graph query path.
• Reuse the existing glob-to-LIKE behavior, including substring matching when no wildcard is provided.
• Add a regression test for issue search_graph: file_pattern returns unfiltered results on 0.8.1 (follow-up to #200) #552 showing BM25 results exclude matching symbols outside the requested path.

Fixes #552

Validation

• git diff --check
• make -f Makefile.cbm build/c/test-runner
• build/c/test-runner | awk 'BEGIN{show=0} /^=== mcp ===/{show=1} /^=== language ===/{exit} show{print}'
• Actual CLI usage with patched checkout binary and isolated CBM_CACHE_DIR:
  • Indexed a throwaway repo containing both src/lib/status.c and src/components/status.c.
  • search_graph with query:"status" returned both BM25 results.
  • search_graph with query:"status", file_pattern:"src/lib/*" returned only src/lib/status.c with total:1.