feat: add table branch error codes across spec, python, and java

[brendanclement]

Description

The Rust implementation in lance-format/lance defines TableBranchNotFound (22) and TableBranchAlreadyExists (23), but the spec registry and the Python and Java interface modules stop at 21. Clients converting errors by code (for example pylance via lance_namespace.errors.from_error_code) currently reclassify branch failures as InternalError, so deleting a missing branch surfaces as an internal error instead of a branch-not-found error.

This adds the two branch error codes everywhere the registry lives:

• spec.yaml: error code registry gains 22 and 23, plus 21 (Throttling), which was already present in every implementation but missing from the spec list.
• Python: ErrorCode members, TableBranchNotFoundError / TableBranchAlreadyExistsError, from_error_code mapping, and package re-exports.
• Java (core and core-async): ErrorCode entries, ErrorFactory cases, and the two exception classes in each module, mirroring the existing tag exceptions.

Testing

Python round-trip test for both codes (from_error_code returns the typed exceptions with the right code values).

Surfaced by review feedback on lance-format/lance#7219, where the new Python branch API bindings expose the gap.