test(techdemo): use handle field constant

Remove the last literal __h wire spelling from techdemo E2E tests, including the unknown-handle negative case.

req: dx/003

req: examples/003

examples/techdemo/tests/e2e.rs

@@ -257,7 +257,7 @@ fn product_is_e2e_working_over_http() {
     assert!(sse.text().contains("event: slhx"));
     assert!(sse.text().contains("data: "));
 
-    let unknown = post("/", "__h=999999");
+    let unknown = post("/", &format!("{SLHX_HANDLE_FIELD}=999999"));
     assert_eq!(unknown.status, 404);
     assert!(unknown.text().contains("unknown slhx handle id 999999"));
 }