Test failures with click 8.2.0

Start with tests/test_embed_cli.py, especially test_embed_multi_files_encoding and its CliRunner setup. Read the Click 8.2.0 changes linked in the issue, then run the failing test; done means the affected tests pass with Click 8.2.0.

While building for nixpkgs:

______________ test_embed_multi_files_encoding[extra_args0-None] _______________

multi_files = ('/nix/var/nix/builds/nix-build-python3.13-llm-0.27.1.drv-0/b/pytest-of-_nixbld13/pytest-0/test_embed_multi_files_enco...ix/builds/nix-build-python3.13-llm-0.27.1.drv-0/b/pytest-of-_nixbld13/pytest-0/test_embed_multi_files_encodin0/files'))
extra_args = [], expected_error = None

    @pytest.mark.parametrize(
        "extra_args,expected_error",
        (
            # With no args default utf-8 with latin-1 fallback should work
            ([], None),
            (["--encoding", "utf-8"], "Could not decode text in file"),
            (["--encoding", "latin-1"], None),
            (["--encoding", "latin-1", "--encoding", "utf-8"], None),
            (["--encoding", "utf-8", "--encoding", "latin-1"], None),
        ),
    )
    def test_embed_multi_files_encoding(multi_files, extra_args, expected_error):
        db_path, files = multi_files
>       runner = CliRunner(mix_stderr=False)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
E       TypeError: CliRunner.__init__() got an unexpected keyword argument 'mix_stderr'

tests/test_embed_cli.py:585: TypeError
__ test_embed_multi_files_encoding[extra_args1-Could not decode text in file] __

multi_files = ('/nix/var/nix/builds/nix-build-python3.13-llm-0.27.1.drv-0/b/pytest-of-_nixbld13/pytest-0/test_embed_multi_files_enco...ix/builds/nix-build-python3.13-llm-0.27.1.drv-0/b/pytest-of-_nixbld13/pytest-0/test_embed_multi_files_encodin1/files'))
extra_args = ['--encoding', 'utf-8']
expected_error = 'Could not decode text in file'

See https://click.palletsprojects.com/en/stable/changes/#version-8-2-0