Refactor RecordsEndpoint validation and security logic

[fderuiter]

This PR refactors RecordsEndpoint in imednet/endpoints/records.py to address "Primitive Obsession" and "Leaky Abstraction" smells.

Changes:

1. imednet/utils/security.py: Added validate_header_value function to check for newline characters in header values.
2. imednet/validation/cache.py: Added validate_record_entry function to validate a single record entry against the schema, handling various form key aliases (formKey, form_key, formId, form_id).
3. imednet/endpoints/records.py: Refactored RecordsEndpoint to use the extracted helpers. _validate_records_if_schema_present now delegates to validate_record_entry, and _build_headers uses validate_header_value.
4. tests/unit/endpoints/test_records_endpoint.py: Added a test case test_create_raises_on_header_injection to verify the security check.

These changes adhere to the "Architect" manifesto by simplifying the endpoint logic, enforcing strict typing, and improving code organization.

---

PR created automatically by Jules for task 8985655797373997743 started by @fderuiter

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.