feat: add table branch operations and branch parameter

[brendanclement]

Summary

Adds first-class table branch support to the namespace spec: three branch lifecycle operations plus an optional branch parameter on the branch-scoped table operations. Branches are already a first-class ref in the Lance table format (alongside tags and versions), and this brings the namespace API in line with that model. An absent branch always means the main branch, so the change is fully backward compatible.

New operations

• CreateTableBranch (POST /v1/table/{id}/branches/create): name is required; optional from_branch and from_version create the branch from a specific point (defaulting to the latest version of main).
• ListTableBranches (POST /v1/table/{id}/branches/list): returns a map of branch name to BranchContents.
• DeleteTableBranch (POST /v1/table/{id}/branches/delete): name is required.

New BranchContents schema (modeled on TagContents): parentBranch (absent when the branch came from main), parentVersion, createAt, manifestSize, and optional metadata.

The branch parameter

A branch selects which ref a branch-scoped operation targets, so it is the natural sibling of the existing version parameter and is carried wherever each operation already passes its parameters:

• Operations with a JSON request body get a branch field in the body, placed alongside version where the operation already has one (describe, count, query, stats, update, delete, the alter and backfill-column operations, the index operations, the version operations, and the tag operations).
• Operations with no JSON parameter body get branch as a query parameter, the same way delimiter is already passed:
  • Arrow-stream bodies: InsertIntoTable, MergeInsertIntoTable
  • bodyless or raw-body: ListTableVersions, UpdateTableSchemaMetadata, DropTableIndex

[jackye1995]

looks good to me!