fix: Include default_branch in project export/import

What does this MR do and why?

fix: Include default_branch in project export/import

Add default_branch to export configuration and implement setter method to preserve the default branch setting across project export/import operations. This addresses the issue where default_branch was overlooked when the allowlist approach was introduced in GitLab 14.5.

Changes include:

• Added default_branch to import_export.yml methods section
• Implemented default_branch= setter in HasRepository concern
• Added safe error handling to prevent import failures

References

Screenshots or screen recordings

Source	Before	After

How to set up and validate locally

• In a project, create a branch with a name later in the alphabetical order then the default branch.
• Export the project via Settings > General > Advanced > Export Project
• Refresh the page until the file is available to download then download it
• Import the project New Project > Import Project > Gitlab Import >
• Observe incorrect default branch assignment
• Check out into the 342246-include-default-branch-config
• Re-export the project
• Import the project
• Observe correct default branch assignment

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Related to #342246