Narrow CodeQL paths-ignore to only fixture directories

Keep test/ and spec/ in CodeQL analysis for better SAST coverage.
Only exclude directories containing intentionally unparseable files
such as UTF-16 encoded fixtures and bad syntax test fixtures.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hsbt

.github/codeql/codeql-config.yml

@@ -1,5 +1,6 @@
 paths-ignore:
   - benchmark
   - sample
-  - spec
-  - test
+  - spec/ruby/command_line/fixtures
+  - spec/ruby/core/exception/fixtures
+  - spec/ruby/language/fixtures