Pull Requests: More tests and adjustments

[N-o-Z]

Closes #8211

Change Description

Background

Added some unit tests in graveler. Modified CreatePullRequest endpoint to return an object

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:recycle: PR Preview c7a787686e0296d076b160690a2bde9abce9a6ab has been successfully destroyed since this PR has been closed.

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E2E Test Results - DynamoDB Local - Local Block Adapter

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E2E Test Results - Quickstart