test(v0): check auth and wizard form handlers

Annotate the v0 auth and wizard form models with #[slhx::form], make their templates expose checked form fields, and add handler smoke tests over the generated form resources.

req: form/001

req: form/004

req: form/006

req: derive_handler/003

examples/v0/templates/auth.heml

@@ -1,7 +1,7 @@
 <section data-slhx-root="auth">
   <form data-slhx-handle="login" data-slhx-form="credentials">
-    <input name="email" type="email" autocomplete="username">
-    <input name="password" type="password" autocomplete="current-password">
+    <input name="email" type="email" autocomplete="username" required>
+    <input name="password" type="password" autocomplete="current-password" required>
     <button type="submit">Sign in</button>
   </form>
   <p data-slhx-slot="login_status">Signed out</p>

examples/v0/templates/wizard.heml

@@ -1,5 +1,6 @@
 <section data-slhx-root="wizard">
   <form data-slhx-handle="next_step" data-slhx-form="wizard_input">
+    <input type="hidden" name="step" value="1" required>
     <div data-slhx-slot="wizard_step">Step 1</div>
     <button type="submit">Next</button>
   </form>