feat: auto-detect provider when switching models via /model (#1506)

When typing /model deepseek-chat while on a different provider, the
model name now auto-resolves to the correct provider instead of
silently staying on the wrong one and causing API errors.

Detection priority:
1. Direct provider with credentials (e.g. DEEPSEEK_API_KEY set)
2. OpenRouter catalog match with proper slug remapping
3. Direct provider without creds (clear error beats silent failure)

Also adds DeepSeek as a first-class API-key provider — just set
DEEPSEEK_API_KEY and /model deepseek-chat routes directly.

Bare model names get remapped to proper OpenRouter slugs:
  /model gpt-5.4 → openai/gpt-5.4
  /model claude-opus-4.6 → anthropic/claude-opus-4.6

Salvages the concept from PR #1177 by @virtaava with credential
awareness and OpenRouter slug mapping added.

Co-authored-by: virtaava <virtaava@users.noreply.github.com>

tests/test_cli_model_command.py

@@ -64,8 +64,8 @@ class TestModelCommand:
             cli_obj.process_command("/model gpt-5.4")
 
         output = capsys.readouterr().out
-        # Model is accepted (with warning) even if not in API listing
-        assert cli_obj.model == "gpt-5.4"
+        # Auto-detection remaps bare model names to proper OpenRouter slugs
+        assert cli_obj.model == "openai/gpt-5.4"
 
     def test_validation_crash_falls_back_to_save(self, capsys):
         cli_obj = self._make_cli()