Refactor IGiteaMock.cs to use async/await patterns consistently

[coderabbitai]

Several methods in src/Designer/backend/tests/Designer.Tests/Mocks/IGiteaMock.cs currently return completed tasks using Task.FromResult or block on .Result, rather than using proper async/await patterns. This can hide async faults and doesn't follow best practices for async code.

The file should be refactored to use async/await consistently throughout, ensuring exceptions and cancellation tokens propagate correctly.

Context:

• PR: feat: update org library endpoints #16948
• Comment: feat: update org library endpoints #16948 (comment)
• Requested by: @HauklandJ