test(examples): forbid literal handle field posts

Extend canonical example contracts to catch raw __h= request bodies in tests and docs, keeping examples on the axum handle-field constant and generated handles.

req: dx/003

req: examples/003

slhx-test/tests/examples_contract.rs

@@ -68,6 +68,7 @@ fn canonical_examples_do_not_author_low_level_resource_plumbing() {
         "register_handle(",
         "RenderSlotExt",
         ".render(&",
+        "__h=",
         "name=\"__h\"",
         "name='__h'",
     ];