#21140: test(security): add regression test for claude-opus-4-6 audit

Fixes #10521. ## Description This PR adds a regression test to `src/security/audit.test.ts` to ensure that `anthropic/claude-opus-4-6` is correctly identified as a strong/safe model in security audits. ## Verification - Verified manually with `pnpm test src/security/audit.test.ts`. - Confirmed existing regex handles the version string correctly. <!-- greptile_comment --> <h3>Greptile Summary</h3> Adds regression test verifying that `anthropic/claude-opus-4-6` is correctly recognized as a strong model (>= Claude 4.5) by the security audit system. The test confirms that the existing regex pattern in `src/security/audit-extra.sync.ts:162` properly matches the `claude-opus-4-6` version format and doesn't flag it with a "Below Claude 4.5" warning in the `models.hygiene` check. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with no risk - This is a pure test addition that adds regression coverage for `claude-opus-4-6` model detection. The test follows existing patterns in the file, uses the same test structure as similar tests (lines 1189-1214), and verifies expected behavior without modifying any production code. The regex pattern in `audit-extra.sync.ts` already correctly handles the version format. - No files require special attention _{Last reviewed commit: bd7990e} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->