Resolve "Bug - Web Based IDE - The "Merge" Requests Selection displays Merge Requests from other projects"

What does this MR do?

Limits the search of the default MR dropdown in WebIDE to be scoped to the current project.

What are the relevant issue numbers?

Closes #50839 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.