Update MCP to reuse branch for SDK generation

[maririos]

With the new feature added into the SDK generation pipeline (#11460) the Release Planner and the MCP need to adopt this feature to avoid having multiple SDK PRs for the same API spec PR.

For the release planner, ideally, we can just use the CLI

• Check if release plan already has a pull request for the language
• Get head branch name from previous pull request if pull request is not in closed or merged status.
• Pass the branch name to SDK generation pipeline.