test: check request errors and stop asserting PathRewrite on /v1/models

/v1/models is served from config by ServeHTTP (task 2.5) so it never reaches
routing; the rewrite test now uses a non-reserved path.

internal/proxy/headers_test.go

@@ -71,7 +71,7 @@ func TestHeaderHygiene(t *testing.T) {
 			t.Errorf("hop-by-hop header %s should be stripped, but found: %s", header, receivedHeaders.Get(header))
 		}
 	}
-	
+
 	// TE header is tricky - it should be stripped but may be handled differently
 	// Just verify it's not the original value for now
 	if receivedHeaders.Get("TE") == "trailers" {