refactor(core): add LLMClient facade trait; decouple TUI from Provider/MCP details

crates/owlen-core/src/facade/llm_client.rs

@@ -0,0 +1,32 @@
+use std::sync::Arc;
+
+use async_trait::async_trait;
+
+use crate::{
+    Result,
+    llm::ChatStream,
+    mcp::{McpToolCall, McpToolDescriptor, McpToolResponse},
+    types::{ChatRequest, ChatResponse, ModelInfo},
+};
+
+/// Object-safe facade for interacting with LLM backends.
+#[async_trait]
+pub trait LlmClient: Send + Sync {
+    /// List the models exposed by this client.
+    async fn list_models(&self) -> Result<Vec<ModelInfo>>;
+
+    /// Issue a one-shot chat request and wait for the complete response.
+    async fn send_chat(&self, request: ChatRequest) -> Result<ChatResponse>;
+
+    /// Stream chat responses incrementally.
+    async fn stream_chat(&self, request: ChatRequest) -> Result<ChatStream>;
+
+    /// Enumerate tools exposed by the backing provider.
+    async fn list_tools(&self) -> Result<Vec<McpToolDescriptor>>;
+
+    /// Invoke a tool exposed by the provider.
+    async fn call_tool(&self, call: McpToolCall) -> Result<McpToolResponse>;
+}
+
+/// Convenience alias for trait-object clients.
+pub type DynLlmClient = Arc<dyn LlmClient>;