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/bodysize_test.go

@@ -323,7 +323,10 @@ func TestBodySizeCapRejectionLogged(t *testing.T) {
 
 	// Send an oversized body
 	body := strings.Repeat("a", int(maxBodySize)+1)
-	resp, _ := http.Post(server.URL+"/test", "text/plain", strings.NewReader(body))
+	resp, err := http.Post(server.URL+"/test", "text/plain", strings.NewReader(body))
+	if err != nil {
+		t.Fatalf("request failed: %v", err)
+	}
 	resp.Body.Close()
 
 	// Verify rejection status