test(branch): remove obsolete test_branch.py after branch module refactor

The old test tests/unit/pkgmgr/actions/test_branch.py has been removed because:

- it targeted the previous monolithic pkgmgr.actions.branch module structure
- its patch targets no longer match the refactored code
- its responsibilities are now fully covered by the new, dedicated unit,
  integration, and E2E tests for branch actions and CLI wiring

This avoids redundant coverage and prevents misleading or broken tests
after the branch refactor.

https://chatgpt.com/share/693bcc8d-b84c-800f-8510-8d6c66faf627

tests/unit/pkgmgr/actions/branch/__init__.py

@@ -0,0 +1,33 @@
+import unittest
+from unittest.mock import patch, MagicMock
+
+from pkgmgr.actions.branch.utils import _resolve_base_branch
+from pkgmgr.core.git import GitError
+
+
+class TestResolveBaseBranch(unittest.TestCase):
+    @patch("pkgmgr.actions.branch.utils.run_git")
+    def test_resolves_preferred(self, run_git):
+        run_git.return_value = None
+        result = _resolve_base_branch("main", "master", cwd=".")
+        self.assertEqual(result, "main")
+        run_git.assert_called_with(["rev-parse", "--verify", "main"], cwd=".")
+
+    @patch("pkgmgr.actions.branch.utils.run_git")
+    def test_resolves_fallback(self, run_git):
+        run_git.side_effect = [
+            GitError("main missing"),
+            None,
+        ]
+        result = _resolve_base_branch("main", "master", cwd=".")
+        self.assertEqual(result, "master")
+
+    @patch("pkgmgr.actions.branch.utils.run_git")
+    def test_raises_when_no_branch_exists(self, run_git):
+        run_git.side_effect = GitError("missing")
+        with self.assertRaises(RuntimeError):
+            _resolve_base_branch("main", "master", cwd=".")
+
+
+if __name__ == "__main__":
+    unittest.main()