feat(datafusion): support the create_branch procedure

[jackylee-ch]

Java exposes sys.create_branch(table, branch[, tag][, ignore_if_exists]) to fork a branch off the current schema or a tag without touching main. The core BranchManager::create_branch / create_branch_from_tag primitives already existed (#290/#482 wired the branch reader and manager); only the DataFusion CALL procedure was missing.

This registers create_branch: it declares the table/branch/tag/ignore_if_exists parameters so a misspelled argument is rejected (matching Java's binding), and dispatches to BranchManager — creating from a tag when one is given, and treating an existing branch as a no-op only when ignore_if_exists is set.

Tested end-to-end: CALL, then assert the branch appears in $branches; recreating errors; recreating with ignore_if_exists is a no-op.

[JingsongLi]

Requirement fit: SUPPORTED. Normal branch creation, duplicate handling, and a branch created from a tag worked in local DataFusion SQL tests. However, P1: reject path separators in branch names. The shared BranchManager::validate_branch_name allows b1/hidden. I reproduced this at 697e6c9: after creating b1, CALL sys.create_branch(table => 'test_db.t1', branch => 'b1/hidden') succeeds, but SELECT * FROM t1$branches WHERE branch_name = 'b1/hidden' returns zero rows. The branch is created underneath branch-b1 and is invisible to normal branch listing. Please validate names at the shared manager boundary before writing any schema/tag/snapshot files, and add a SQL regression confirming an invalid name fails without creating files. Original PR test, a temporary from-tag test, formatting and diff checks pass; CI is green but misses this case. Temporary test edits were restored.

[jackylee-ch]

Addressed. validate_branch_name now rejects / and \ at the shared BranchManager boundary, so create_branch and create_branch_from_tag fail before copying any schema, tag or snapshot file. Added the SQL regression: after creating b1, CALL sys.create_branch(..., branch => 'b1/hidden') fails with the path-separator error, nothing is created under branch-b1/, and $branches still lists only b1. Rebased onto main.