[router]restructure protocol modules for better organization (#9321)

sgl-router/src/protocols/openai/completions/mod.rs

@@ -0,0 +1,10 @@
+// Completions API module (v1/completions)
+
+pub mod request;
+pub mod response;
+
+// Re-export main types for convenience
+pub use request::CompletionRequest;
+pub use response::{
+    CompletionChoice, CompletionResponse, CompletionStreamChoice, CompletionStreamResponse,
+};