fix: skip model auto-detection for custom/local providers

When the user is on a custom provider (provider=custom, localhost, or
127.0.0.1 endpoint), /model <name> no longer tries to auto-detect a
provider switch. The model name changes on the current endpoint as-is.

To switch away from a custom endpoint, users must use explicit
provider:model syntax (e.g. /model openai-codex:gpt-5.2-codex).
A helpful tip is printed when changing models on a custom endpoint.

This prevents the confusing case where someone on LM Studio types
/model gpt-5.2-codex, the auto-detection tries to switch providers,
fails or partially succeeds, and requests still go to the old endpoint.

Also fixes the missing prompt_toolkit.auto_suggest mock stub in
test_cli_init.py (same issue already fixed in test_cli_new_session.py).

tests/test_cli_init.py

@@ -42,6 +42,7 @@ def _make_cli(env_overrides=None, config_overrides=None, **kwargs):
         "prompt_toolkit.key_binding": MagicMock(),
         "prompt_toolkit.completion": MagicMock(),
         "prompt_toolkit.formatted_text": MagicMock(),
+        "prompt_toolkit.auto_suggest": MagicMock(),
     }
     with patch.dict(sys.modules, prompt_toolkit_stubs), \
          patch.dict("os.environ", clean_env, clear=False):