[python] Add branch CRUD REST implementation to RESTCatalog (#7747) `pypaimon`'s `Catalog` ABC has stubs for `create_branch` / `drop_branch` / `fast_forward` / `list_branches` raising `NotImplementedError`, and is missing `rename_branch` entirely. The `RESTCatalog` never overrode any of them, so calling any branch operation on a real REST catalog raises `NotImplementedError` instead of issuing the REST call. Java defines all five (`paimon-core/.../catalog/Catalog.java:843-912`) with a complete REST implementation in `paimon-core/.../rest/RESTCatalog.java:703-769`. This PR closes that gap. This is the sister PR of [#7746 — Tag CRUD](https://github.com/apache/paimon/pull/7746); same shape (abstract stub + RESTCatalog overrides + wire DTOs + URL builders + mock REST server route handlers + tests). `FilesystemCatalog` inherits the abstract `NotImplementedError` stubs; a Python-side `BranchManager` port is tracked separately.

Apache Paimon is a lake format that enables building a Realtime Lakehouse Architecture with Flink and Spark for both streaming and batch operations. Paimon innovatively combines lake format and LSM structure, bringing realtime streaming updates into the lake architecture.
Background and documentation are available at https://paimon.apache.org
Paimon's former name was Flink Table Store, developed from the Flink community. The architecture refers to some design concepts of Iceberg. Thanks to Apache Flink and Apache Iceberg.
Paimon tracks issues in GitHub and prefers to receive contributions as pull requests.
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