Split test_branching.py into a tests/branching package by area #950

Proposed change

Split tests/test_branching.py (about 4,000 lines) into a tests/branching/ package by what the tests exercise:

• base: the shared helpers and BranchingTestBase (HAS_BRANCHING, _provision_branch, _close_branch_connections and so on).
• test_merge_matrix: the merge-and-revert tests run under both merge strategies (BaseBranchingTests and its iterative and squash subclasses), and MissingFieldTypesTestCase.
• test_merge: the other merge scenarios: field attribute changes, tags and journal entries, choice sets and search weight, and concurrent edits merged under both strategies.
• test_sync: BranchSyncTestCase and ConcurrentEditSyncTestCase.
• test_renames: SequentialRenameTestCase and its squash subclass.
• test_heal: BranchUpgradeHealTestCase.
• test_graphql: GraphQLBranchIsolationTestCase and GraphQLBranchEndpointTestCase.
• test_isolation: branch deletion and isolation from main (BranchDeletionTestCase, BranchContextDeletionTestCase, InboundRelationBranchIsolationTestCase, BranchCOTDeletionIsolationTestCase, PolymorphicReverseAccessorBranchTestCase).
• test_helpers: tests/test_branching_helpers.py, moved.

tests/schema/test_apply_permissions.py imports its branch helpers from branching.base instead. Tests move unchanged.

Justification

test_branching.py is the largest test module by far. Its classes cover merge, sync, upgrade heal, GraphQL and isolation, so a failing test is hard to place and a new one has no obvious home. A package split by area matches tests/api/, tests/models/, tests/upgrade/ and tests/graphql/.