[router][protocols] Add Axum validate extractor and use it for /v1/chat/completions endpoint (#11588)

sgl-router/tests/spec/mod.rs

@@ -0,0 +1,8 @@
+// Protocol specification tests
+// These tests were originally in src/protocols/spec.rs and have been moved here
+// to reduce the size of that file and improve test organization.
+
+mod chat_completion;
+mod chat_message;
+mod embedding;
+mod rerank;