Add OpenAI/LiteLLM provider support with settings UI

- Add openai/ provider module with translator, client, convert, request/response translators
- Add shared provider/ types (ConversationMessage, MessageRole, ProviderConfig enum)
- Wire OpenAI-compatible provider dispatch alongside Bedrock in response_stream.rs
- Add ai.openai.* settings (enabled, base_url, api_key, model, models)
- Add OpenAI/LiteLLM settings page with model fetch, picker, and config UI
- Extend model menu items and llms.rs to surface LiteLLM models
- Update WARP.md with OpenAI provider architecture docs

app/src/ai/bedrock/models_tests.rs

@@ -92,6 +92,7 @@ fn test_get_effective_models_custom_overrides() {
         model_id: "custom.model-v1:0".to_string(),
         display_name: "Custom Model".to_string(),
         vision_supported: false,
+        context_size: 200_000,
     }];
     let models = get_effective_models(&custom);
     assert_eq!(models.len(), 1);