feat(providers): add native tool-call API support via chat_with_tools

Add chat_with_tools() to the Provider trait with a default fallback to
chat_with_history(). Implement native tool calling in OpenRouterProvider,
reusing existing NativeChatRequest/NativeChatResponse structs. Wire the
agent loop to use native tool calls when the provider supports them,
falling back to XML-based parsing otherwise.

Changes are purely additive to traits.rs and openrouter.rs. The only
deletions (36 lines) are within run_tool_call_loop() in loop_.rs where
the LLM call section was replaced with a branching if/else for native
vs XML tool calling.

Includes 5 new tests covering:
- chat_with_tools error path (missing API key)
- NativeChatResponse deserialization (tool calls only, mixed)
- parse_native_response conversion to ChatResponse
- tools_to_openai_format schema validation

src/providers/traits.rs

@@ -170,6 +170,23 @@ pub trait Provider: Send + Sync {
     async fn warmup(&self) -> anyhow::Result<()> {
         Ok(())
     }
+
+    /// Chat with tool definitions for native function calling support.
+    /// The default implementation falls back to chat_with_history and returns
+    /// an empty tool_calls vector (prompt-based tool use only).
+    async fn chat_with_tools(
+        &self,
+        messages: &[ChatMessage],
+        _tools: &[serde_json::Value],
+        model: &str,
+        temperature: f64,
+    ) -> anyhow::Result<ChatResponse> {
+        let text = self.chat_with_history(messages, model, temperature).await?;
+        Ok(ChatResponse {
+            text: Some(text),
+            tool_calls: Vec::new(),
+        })
+    }
 }
 
 #[cfg(test)]