CG-17310: Disable PR creation Sentry errors

[codegen-sh]

Motivation

When trying to create a PR on a branch that already has a PR, a Sentry error is generated and the agent is not shown a good error message.

Changes

1. Modified GitRepoClient.create_pull to check if a PR already exists for the branch before attempting to create a new one
2. Updated Codebase.create_pr to use get_or_create_pull instead of create_pull to handle the case when a PR already exists
3. Added a test case to verify that creating a PR on a branch that already has a PR returns the existing PR

Testing

Added a new test case test_codebase_create_pr_existing_pr that verifies the behavior when trying to create a PR on a branch that already has a PR.

Fixes CG-17310

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