feat(chat): I-commit — model→STRING, OLLAMA_HOST docker support, dynamic test fixtures

- OllamaChatCompletion.model changed from COMBO to plain STRING so it can
  receive a wired value from OllamaLoadModel without a separate model_name
  socket (removes dual-input UX confusion and COMBO validation errors)
- Removed model_name optional input — model STRING is the single entry point
- _load_default_models() tries OLLAMA_HOST env var → host.docker.internal →
  localhost so COMBO nodes pre-populate on server start inside Docker
- docker-compose.dev.yml: OLLAMA_HOST=http://host.docker.internal:11434 so
  model dropdowns are populated at startup without a manual Refresh click
- JS: refreshModelWidget handles both COMBO and STRING widget types; keeps
  user-typed/wired value when it is already a known model
- Tests: first_generative_model fixture dynamically selects the first
  installed model instead of hardcoding embeddinggemma:latest (which is an
  embedding-only model that rejects /api/generate with HTTP 400)
- Tests: test_multi_turn_receives_context passes think=False to prevent
  Qwen3 thinking-mode models from returning empty content fields
- Screenshots retaken: ollama_chat.png, ollama_lifecycle.png, ollama_workflow.png

All 80 unit + 9 integration tests pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RWysE6jYn4YjNorLBQ1cXb

docker-compose.dev.yml

@@ -9,6 +9,10 @@ services:
     # On macOS/Windows Docker Desktop: host.docker.internal is built-in.
     extra_hosts:
       - "host.docker.internal:host-gateway"
+    environment:
+      # Tells comfydv where to reach Ollama on the host so model dropdowns are
+      # pre-populated at server start without needing a manual Refresh click.
+      - OLLAMA_HOST=http://host.docker.internal:11434
     command:
       - sh
       - -c