get_pull_requests() support

[maciekbanas]

Part of #754.

Implement get_pull_requests() for BitBucket Cloud:

• Retrieve PRs via GET /2.0/repositories/{workspace}/{repo_slug}/pullrequests with state filter.
• Map to the standard gitstats_pull_requests table: number, created_at, merged_at, state, author, source_branch, target_branch, repo_name, organization, api_url.
• Handle pagination (cursor-based with next URL).
• Unit tests with mocked API responses.