Safe move_to_start and move_to_end

What does this MR do?

This brings the relative positioning improvements from !37724 (merged) back in after being reverted in !39787 (merged), along with fixes for the root cause of the problems seen in #238416 (closed)

At heart, while a lot of operations were made safer, move_to_start and move_to_end did not correctly check the gap size, and raised errors when they could have rebalanced instead.

Current code is also faulty - it unconditionally moves items to the max position even if that position is occupied. This violates the expectation that all other items will have lower max-positions after the operation completes.

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

/cc @engwan