fix: refuse to give the API the mock LLM's port

Both preflights passed for `dev --mock-llm --port 8090`: the port was free, and
then the mock and the API were each handed it. One child could not bind, and the
API was pointed at that port as its model server while trying to listen on it.

tests/deploy/test_doctor.py

@@ -66,6 +66,20 @@ class TestDevCommand:
                 _run(["dev", "--mock-llm", "--port", str(self._free_port())], _context())
         assert started == [], "nothing is spawned when the mock LLM has nowhere to listen"
 
+    def test_the_api_cannot_be_given_the_mock_llm_port(self, monkeypatch, tmp_path):
+        """Both free-port checks pass here: the port is free, and then two children want it."""
+        from docsgpt.deploy import dev as dev_module
+
+        self._checkout(monkeypatch, tmp_path)
+        (tmp_path / "scripts").mkdir()
+        (tmp_path / "scripts" / "mock_llm.py").write_text("", encoding="utf-8")
+        started = []
+        monkeypatch.setattr(dev_module, "run", lambda children, **kwargs: started.append(children) or 0)
+        monkeypatch.setattr(dev_module, "MOCK_LLM_PORT", self._free_port())
+        with pytest.raises(DeployError, match="mock LLM listens"):
+            _run(["dev", "--mock-llm", "--port", str(dev_module.MOCK_LLM_PORT)], _context())
+        assert started == [], "nothing is spawned when the two would collide"
+
     def test_it_runs_the_children_it_planned_and_says_where(self, monkeypatch, tmp_path, capsys):
         from docsgpt.deploy import dev as dev_module