Validate existence of SHAs in diff refs

What does this MR do?

This is a follow-up to #36493 (closed) / https://gitlab.com/gitlab-org/security/gitlab/-/merge_requests/145#note_279033460.

The original issue was that storing a null base_sha in a note on designs makes the design unavailable due to a JS error on the frontend when loading the notes:

• https://gitlab.com/gitlab-org/security/gitlab/-/merge_requests/145 fixes the backend to not return an error for null base_sha values.
• !23491 (merged) fixes the frontend error handling.

This MR now validates all values for base_sha / start_sha / head_sha in all position attributes. There's a lot of complexity around diff notes, so this is implemented behind a feature flag and will be rolled out in steps: #239342 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team