refactor(provider): unify Provider responses with ChatResponse

- Switch Provider trait methods to return structured ChatResponse
- Map OpenAI-compatible tool_calls into shared ToolCall type
- Update reliable/router wrappers and provider tests for new interface
- Make agent loop prefer structured tool calls with text fallback parsing
- Adapt gateway replies to structured responses with safe tool-call fallback

src/providers/mod.rs

@@ -8,7 +8,7 @@ pub mod reliable;
 pub mod router;
 pub mod traits;
 
-pub use traits::{ChatMessage, Provider};
+pub use traits::{ChatMessage, ChatResponse, Provider, ToolCall};
 
 use compatible::{AuthStyle, OpenAiCompatibleProvider};
 use reliable::ReliableProvider;