test(bump): cover _stage_updated_files error path

The git.add return-code check added in the previous reviewer-feedback commit was not exercised by tests. This adds a regression test that mocks git.add returning non-zero and asserts BumpCommitFailedError is raised with a useful message.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: bearomorphism

tests/commands/test_bump_command.py

@@ -13,6 +13,7 @@
 from commitizen import cmd, defaults, git, hooks
 from commitizen.config.base_config import BaseConfig
 from commitizen.exceptions import (
+    BumpCommitFailedError,
     BumpTagFailedError,
     CommitizenException,
     CurrentVersionNotFoundError,
@@ -1409,6 +1410,42 @@ def test_bump_skips_commit_when_no_files_changed(
     add_mock.assert_not_called()
 
 
+def test_bump_raises_when_git_add_fails(
+    mocker: MockFixture, tmp_commitizen_project: Path, util: UtilFixture
+):
+    """Regression test: ``cz bump`` reports a failing ``git.add``."""
+    project_root = tmp_commitizen_project
+    tmp_commitizen_cfg_file = project_root / "pyproject.toml"
+    tmp_commitizen_cfg_file.write_text(
+        "\n".join(
+            [
+                "[tool.commitizen]",
+                'version = "0.1.0"',
+                'tag_format = "v$version"',
+                'version_files = ["pyproject.toml:version"]',
+            ]
+        ),
+    )
+    util.create_file_and_commit("feat: first feature")
+
+    mocker.patch(
+        "commitizen.git.add",
+        return_value=cmd.Command(
+            out="",
+            err="fatal: pathspec did not match any files",
+            stdout=b"",
+            stderr=b"fatal: pathspec did not match any files",
+            return_code=128,
+        ),
+    )
+
+    with pytest.raises(BumpCommitFailedError) as exc_info:
+        util.run_cli("bump", "--yes")
+
+    assert "git.add error" in str(exc_info.value)
+    assert "fatal: pathspec" in str(exc_info.value)
+
+
 def test_is_initial_tag(mocker: MockFixture, tmp_commitizen_project, util: UtilFixture):
     """Test the _is_initial_tag method behavior."""
     # Create a commit but no tags