fix(cli): mirror --version printed an error instead of a version

`main()` rewrites a bare invocation into `serve`, which is what makes
`mirror --port 8055` work at all — but it did that for every argument that
starts with a dash, so `mirror --version` became `mirror serve --version`, the
subparser has no such flag, and the command exited with "unrecognized
arguments". Found by pulling the image CI published and asking it what it was.

The rewrite rule is now `with_serve()`, extracted so the eight interesting
inputs are asserted directly — including `--port 9999` still becoming a serve
and `serve --version` staying as typed. `serve` also grew its own --version,
because typing it there is just as reasonable and argparse does not inherit
flags from a parent parser.

The extraction is not cosmetic: the first version of this test monkeypatched
`cmd_serve` to prove the rewrite, which did nothing, because argparse binds
`func` at parse time — the test would have started a real uvicorn on 9999 and
hung the suite. Asserting the pure function is both correct and safe.

187 tests pass, 2 skipped; ruff and pyright clean.

LLM-Generated: true

tests/test_config.py

@@ -114,6 +114,36 @@ class TestCli:
             "fd00::/64",
         ]
 
+    @pytest.mark.parametrize(
+        ("given", "expected"),
+        [
+            ([], ["serve"]),
+            (["--port", "9999"], ["serve", "--port", "9999"]),
+            (["--allow-cidr", "10.2.0.0/22"], ["serve", "--allow-cidr", "10.2.0.0/22"]),
+            (["serve"], ["serve"]),
+            (["serve", "--version"], ["serve", "--version"]),
+            (["--version"], ["--version"]),
+            (["--help"], ["--help"]),
+            (["-h"], ["-h"]),
+        ],
+    )
+    def test_only_informational_flags_escape_the_implicit_serve(self, given, expected):
+        assert entry.with_serve(given) == expected
+
+    @pytest.mark.parametrize("argv", [["--version"], ["serve", "--version"]])
+    def test_version_prints_and_exits_cleanly(self, argv, capsys):
+        with pytest.raises(SystemExit) as caught:
+            entry.main(argv)
+        assert caught.value.code == 0
+        assert f"mirror {entry.__version__}" in capsys.readouterr().out
+
+    def test_help_lists_the_subcommand_it_is_not_being_rewritten_into(self, capsys):
+        with pytest.raises(SystemExit):
+            entry.main(["--help"])
+        out = capsys.readouterr().out
+        assert "usage: mirror" in out
+        assert "serve" in out
+
     def test_a_bad_allow_cidr_exits_with_a_message_not_a_traceback(self, capsys):
         # Config is parsed before anything binds or imports, so this cannot stall
         # on a stray DEBUGPY in the operator's shell.