feat: extract routes from OpenAPI spec files when Noir fallback is used

[google-labs-jules]

What: Added _fallback_openapi logic in src/websec_validator/extractors/routes.py to parse OpenAPI and Swagger specifications (.json, .yml, .yaml files) when the noir scanner fallback is used.
Why: Applications like VAmPI use a spec-first approach (e.g. openapi3.yml), and static recon failed to find endpoints previously.
Risk: Low. Only runs on specific file extensions without modifying global parser logic, safely handles missing or malformed data inside a try-catch block, and falls back to regex line-by-line parsing if json parsing fails.

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PR created automatically by Jules for task 11964988165294847117 started by @raccioly

[google-labs-jules]

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