feat: auto-detect models from server probe in custom endpoint setup (#4218)

Custom endpoint setup (_model_flow_custom) now probes the server first
and presents detected models instead of asking users to type blind:

- Single model: auto-confirms with Y/n prompt
- Multiple models: numbered list picker, or type a name
- No models / probe failed: falls back to manual input

Context length prompt also moved after model selection so the user sees
the verified endpoint before being asked for details.

All recent fixes preserved: config dict sync (#4172), api_key
persistence (#4182), no save_env_value for URLs (#4165).

Inspired by PR #4194 by sudoingX — re-implemented against current main.

Co-authored-by: Xpress AI (Dip KD) <200180104+sudoingX@users.noreply.github.com>

tests/test_cli_provider_resolution.py

@@ -460,13 +460,16 @@ def test_model_flow_custom_saves_verified_v1_base_url(monkeypatch, capsys):
     )
     monkeypatch.setattr("hermes_cli.config.save_config", lambda cfg: None)
 
-    answers = iter(["http://localhost:8000", "local-key", "llm", ""])
+    # After the probe detects a single model ("llm"), the flow asks
+    # "Use this model? [Y/n]:" — confirm with Enter, then context length.
+    answers = iter(["http://localhost:8000", "local-key", "", ""])
     monkeypatch.setattr("builtins.input", lambda _prompt="": next(answers))
 
     hermes_main._model_flow_custom({})
     output = capsys.readouterr().out
 
     assert "Saving the working base URL instead" in output
+    assert "Detected model: llm" in output
     # OPENAI_BASE_URL is no longer saved to .env — config.yaml is authoritative
     assert "OPENAI_BASE_URL" not in saved_env
     assert saved_env["MODEL"] == "llm"