fix: prevent partial matches for exact token searches

[NishadA05]

Summary

Cherry-picks the exact-token search fix from release-0.5.1 into main.

This prevents exact-looking token queries from returning partial or fuzzy matches when the exact token does not appear in the returned filename or chunk text.

Testing

• Cherry-picked merged release PR fix: prevent partial matches for exact token searches #1937 into main.
• Ran:

uv run python -m py_compile src/services/search_service.py

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit

* **Bug Fixes**
  * Improved search behavior for short or identifier-like queries so results now stay exact and avoid broader unrelated matches.
  * When no exact file matches are found for these queries, search now returns no snippets instead of showing loosely related content.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: f7692283-d709-4c1e-ab15-6846dda05558

📥 Commits

Reviewing files that changed from the base of the PR and between 903af2f and b09843b.

📒 Files selected for processing (1)

• src/services/search_service.py

💤 Files with no reviewable changes (1)

• src/services/search_service.py

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Walkthrough

Adds regex-based token query classification to the search service and updates search result post-processing so token-like queries return no chunks when no exact file matches are found.

Changes

Exact Token Match Filtering

Layer / File(s)	Summary
Token detection and exact-match filtering src/services/search_service.py	Adds the re import, introduces _is_exact_token_query, and extends search_tool filtering to return empty chunks for token-like queries without exact_files.

Estimated code review effort: 2 (Simple) | ~10 minutes

Suggested labels

enhancement, search

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main change: tightening exact-token searches to avoid partial matches.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix-exact-knowledge-search-main

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