Mark merge request as preparing on create

What does this MR do?

When a merge request is created, we set the merge_status to preparing so we don't check its mergeable or mark it as mergeable until it's fully prepared.

This prevents race condition like asynchronous pipeline creation which can lead to inconsistent behavior.

NOTE: This depends on !56074 (merged). That MR needs to be merged first and released in 13.10. This MR needs to be merged in 13.11 once 13.10 is released to ensure multi-version compatibility.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• 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

Related to #291012 (closed)