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/openai/mod.rs

@@ -0,0 +1,13 @@
+pub mod client;
+pub mod convert;
+pub mod request_translator;
+pub mod response_translator;
+pub mod translator;
+
+#[cfg(test)]
+#[path = "convert_tests.rs"]
+mod convert_tests;
+
+#[cfg(test)]
+#[path = "request_translator_tests.rs"]
+mod request_translator_tests;