Refactor/internal pipeline v2

[cak]

Refactor: Internal Pipeline v2

This PR refactors the internal request/response pipeline and establishes the foundation for the upcoming v2.0.1 release.

Why

The previous pipeline evolved incrementally and became harder to reason about, extend, and validate across frameworks.

This refactor focuses on:

• clear data flow
• consistent behavior across integrations
• simpler extension points
• stronger guarantees around header application order

What Changed

• Rebuilt the internal middleware pipeline
• Standardized how security headers are constructed and applied
• Replaced implicit behavior and side effects with explicit stages
• Simplified framework adapters to delegate to a single core pipeline
• Improved separation between configuration, processing, and output

Impact

• No breaking changes expected for standard usage
• More predictable behavior across frameworks
• Improved internal testability
• Easier to extend with new headers, policies, or integrations

Testing

• Updated existing tests to validate the new pipeline behavior
• Added coverage for default header application and edge cases
• Verified consistency across frameworks (e.g., FastAPI / ASGI)

Notes

This is an internal refactor and does not introduce new user-facing features.

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Next

v2.0.1 will be the first official v2 release built on this pipeline.