When a Java record is annotated with Immutables' @Value.Builder
(supported since Immutables 2.11.0), the annotation processor generates
a companion {RecordName}Builder class (e.g. GolfPlayerBuilder for record
GolfPlayer) rather than the traditional Immutable{TypeName} implementation
used for interfaces and abstract classes.

The existing ImmutablesBuilderProvider only looked for @Value.Immutable
and the ImmutableXxx naming convention, so it could not detect builders
for @Value.Builder-annotated records.

This commit adds:
- findBuilderInfoForImmutablesRecord(): detects records annotated with
  @Value.Builder and looks up the companion {RecordName}Builder type
- asImmutableRecordBuilderElement(): constructs the qualified name of
  the generated builder following the {RecordName}Builder convention
- findBuilderInfoFromCompanionBuilder(): constructs BuilderInfo directly
  from the companion class using its public no-arg constructor as the
  builder creation method and findBuildMethods() to locate build().
  Immutables always generates a public no-arg constructor (not a static
  factory) for @Value.Builder on records, mirroring how
  DefaultBuilderProvider handles inner *Builder classes.
- immutablesRecordBuilderTest integration test that exercises end-to-end
  mapping through the Immutables-generated record builder using
  Immutables 2.11.0, registered in MavenIntegrationTest and restricted
  to JDK compiler on Java 16+. Tests include a @nullable field to show
  that the builder's mandatory-field enforcement fires correctly when
  NullValueCheckStrategy.ALWAYS causes MapStruct to skip setting a null
  mandatory field, whereas falling back to the constructor would silently
  accept null.

ElementKind.RECORD was added in Java 16; the implementation uses
typeElement.getKind().name() string comparison to remain compilable
on Java releases that pre-date records.

Adds unit tests for the new ImmutablesBuilderProvider record-builder
support, covering the code paths not exercised by integration tests:

- Value.java: minimal stub of org.immutables.value.Value (with both
  @immutable and @builder inner annotations) so that the test
  compilation activates ImmutablesBuilderProvider as the default
  builder provider without requiring the Immutables jar
- GolfPlayer: a @Value.Builder-annotated record with a nullable Integer
  handicap field and mandatory name/age fields
- GolfPlayerBuilder: hand-crafted stand-in for the Immutables-generated
  companion builder, using an initBits bitmask to enforce mandatory
  fields at build() time
- ImmutablesRecordBuilderTest: three @ProcessorTest(Compiler.JDK) tests
  covering the happy path, nullable field handling, and mandatory-field
  enforcement via NullValueCheckStrategy.ALWAYS