Add @FormPrefix for nested form bean binding

[ah3nan]

Problem

Form beans (@Form/@BeanParam) only supported flat form parameters. Nested objects had to be flattened manually, and RESTEasy-style prefixed binding (invoice.street, shipping.street) had no support.

Feature

Adds a new @FormPrefix(String value) annotation (FIELD target) for a field/component on a @Form/@BeanParam bean. The nested object is populated from form parameters prefixed with the given value:

public class Person {
  @FormParam("name")     public String name;
  @FormPrefix("invoice") public Address invoice;
  @FormPrefix("shipping") public Address shipping;
}
public class Address {
  @FormParam("street") public String street;
  @FormParam("zip")    private String postalCode;  // setter-based
  @FormPrefix("contact") public ContactForm contact; // deep nesting
}

submitted as name=bill&invoice.street=Main+St&invoice.zip=12345&invoice.contact.phone=555....

Records are supported too — nested @FormPrefix records are built via their canonical constructor and passed into the parent constructor:

public record Customer(String firstName, List<String> lastName, @FormPrefix("invoice") InvoiceForm invoice) {}

Changes

• avaje-http-api: new io.avaje.http.api.FormPrefix annotation.
• http-generator-core:
  • ParamType: new FORMNESTED.
  • ElementReader: detect @FormPrefix, apply the dotted prefix to form lookups (formParamName()), recursive imports for nested types, form-bean var/constructor wiring.
  • BeanParamReader: recursive nested population — constructor-based for records / single-constructor types, no-arg + setter/field for POJOs; unique local variable names so repeated field names under different prefixes don't collide; nested constructor params are built before the parent.
• All platforms share the core logic; nested reads use each platform's existing form-param access (Helidon, Javalin, Vert.x, Jex, Sigma).

Generated code (Helidon, record example)

var nested = new Customer.InvoiceForm(
    formParams.contains("invoice.street") ? formParams.get("invoice.street") : null,
    formParams.contains("invoice.city") ? formParams.get("invoice.city") : null);

var customer = new Customer(
    formParams.contains("firstName") ? formParams.get("firstName") : null,
    list(Object::toString, formParams.all("lastName", () -> java.util.List.of())),
    nested);

Verification

• New integration tests (exact-body assertions: full binding, partial submit → nulls, repeated-field-name collision, records) added to all 5 test modules: test-nima (Helidon), test-javalin, test-jex, test-vertx (each with FormPrefixControllerTest + RecordFormControllerTest), plus explicit assertions in test-sigma's SigmaProcessorTest.
• tests/test-vertx/src/test/resources/expectedOpenApi.json updated for the new /formprefix and /recordform routes.
• mvn clean package -Ptest → BUILD SUCCESS across all modules.

Notes / future work

• Nested beans must be constructible from their form fields (no-arg + setters/fields for POJOs, or a matching constructor/record).
• Lists/maps of beans (telephoneNumbers[0].countryCode, address[INVOICE].street) are a follow-up (needs runtime key enumeration via Parameters.names()).

[SentryMan]

@rbygrave seems most of the changes are just tests, so it's not that bad to review

[rbygrave]

Hi @ah3nan @SentryMan - I have added some commits with a failing test case for @FormPrefix("customer-info"), and associated fix (to ensure valid java identifier is used).

The change to ensure valid java identifier converts unsupported chars like hyphen - to underscore _. This could technically produce duplicate variable names but I think thats super unlikely and I prefer to keep BeanParamReader.toJavaIdentifier() simple.

Please review those last commits and post back if you are happy with those.

Thanks, Rob.

[rbygrave]

FYI: The failed build there was because this PR lost the avoidTypeShadowing() change (probably due to ordering of the PRs) and I added a test case to exercise the avoidTypeShadowing() - so that was the failing test, and hence that commit to restore the avoidTypeShadowing() change.