[router] Raise chat body cap to 5 MiB for long contexts (#28742)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

experimental/sgl-router/tests/proxy/chat_routing.rs

@@ -10,6 +10,7 @@ use sgl_router::policies::factory::build_registry_with_defaults as build_policy_
 use sgl_router::proxy::Proxy;
 use sgl_router::server::app::build_router;
 use sgl_router::server::app_context::AppContext;
+use sgl_router::server::routes::chat::MAX_CHAT_BODY_BYTES;
 use sgl_router::tokenizer::TokenizerRegistry;
 use sgl_router::workers::{Worker, WorkerRegistry};
 
@@ -514,8 +515,9 @@ async fn oversized_request_body_returns_413() {
     let ctx = build_ctx_with_worker(&worker.url);
     let app = build_router(ctx);
 
-    // 2 MiB body — the configured limit is 1 MiB.
-    let big = vec![b'x'; 2 * 1024 * 1024];
+    // One byte over the configured cap, so the test tracks the cap
+    // (`MAX_CHAT_BODY_BYTES`) instead of a hardcoded size.
+    let big = vec![b'x'; MAX_CHAT_BODY_BYTES + 1];
     let req = Request::builder()
         .method("POST")
         .uri("/v1/chat/completions")