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/request_translator.rs

@@ -992,7 +992,12 @@ pub fn extract_tools(request: &api::Request) -> Vec<ToolDefinition> {
     // Filter out suggest_next_prompt — its action executor waits on a oneshot
     // channel for UI interaction that never fires in the Bedrock path, causing
     // the conversation to stay InProgress forever.
-    tools.retain(|t| t.name != "suggest_next_prompt");
+    // Filter out start_agent/send_message_to_agent — sub-agents are disabled.
+    tools.retain(|t| {
+        t.name != "suggest_next_prompt"
+            && t.name != "start_agent"
+            && t.name != "send_message_to_agent"
+    });
 
     tools
 }