ProviderBase.readFrom() fails to advance past START_ARRAY for MappingIterator with JSON array input #69

Description

ProviderBase.readFrom() does not handle JSON array input when the entity type is MappingIterator. The method calls p.nextToken() which positions the parser at START_ARRAY, then passes it to reader.readValues(p). However, readValues(JsonParser) creates a MappingIterator with managedParser=false, which does not auto-skip START_ARRAY.

The ObjectReader.readValues(JsonParser) Javadoc explicitly states:

for wrapped sequences, parser MUST NOT point to the surrounding START_ARRAY but rather to the token following it.

This means JSON array input like [{"x":1}, {"x":2}] fails with a MismatchedInputException, while NDJSON input like {"x":1}\n{"x":2} works fine.

Steps to Reproduce

Create a Jakarta REST resource method with a MappingIterator parameter:

@POST
@Consumes(MediaType.APPLICATION_JSON)
@Produces(MediaType.APPLICATION_JSON)
public List<Point> process(MappingIterator<Point> points) {
  List<Point> result = new ArrayList<>();
  while (points.hasNext()) {
      result.add(points.next());
  }
  return result;
}

POST a JSON array:

Content-Type: application/json

[{"x": 1, "y": 2}, {"x": 3, "y": 4}]

Expected: Successfully iterates over the two Point objects.

Actual: Throws MismatchedInputException: Cannot deserialize value of type Point from Array value (token START_ARRAY).

Root Cause

In ProviderBase.readFrom():

if (p == null || p.nextToken() == null) {  // positions at START_ARRAY for array input
  // ...
}
// ...
if (multiValued) {
  return reader.readValues(p);  // passes parser still at START_ARRAY
}

readValues(JsonParser) calls _newIterator(p, ctxt, deser, false) — the false means managedParser=false, so MappingIterator's constructor does not skip START_ARRAY:

if (managedParser && p.isExpectedStartArrayToken()) {
  p.clearCurrentToken();  // only reached when managedParser=true
}

The existing test (testMappingIterator in SimpleEndpointTestBase) only sends NDJSON, so this path was never exercised.

Suggested Fix

Advance past START_ARRAY before calling readValues(p):

if (multiValued) {
  if (p.currentToken() == JsonToken.START_ARRAY) {
      p.nextToken();
  }
  return reader.readValues(p);
}