fix(discovery): use pattern-based filtering when tests_root overlaps source

[Saga4]

Summary

• Fixed critical bug where all functions were incorrectly filtered as "test functions" when tests_root equals module_root or project_root
• Introduced pattern-based test file detection that checks filename patterns (.test., .spec., _test., _spec.) and directory patterns (/test/, /tests/, /__tests__/)
• Directory patterns are only checked within the project root to avoid false positives from parent directories (e.g., when repo is located in /path/tests/my-project)

Problem

When auto-detection sets tests_root to the same value as module_root or project_root (common in monorepos and projects without dedicated test directories), the filter condition file_path.startswith(tests_root + os.sep) matches ALL source files, causing them all to be filtered out as "test functions".

Solution

The fix adds an is_test_file() helper function that:

1. Detects when tests_root overlaps with source directories
2. Uses pattern-based matching in this case instead of directory-based matching
3. Falls back to the original behavior when tests are in a dedicated separate directory

Test Plan

• Verified that n8n repo now discovers 1+ optimizable functions instead of 0
• Run existing test suite to ensure no regressions

[misrasaurabh1]

Please add tests