Fix field projection collapsing nested JsonPointers to leaf names

[vharseko]

Summary

Resources.filterResource(JsonValue, Collection<JsonPointer>) built the
projected result using field.leaf() as a flat key:

final String key = field.leaf();
filtered.put(key, value.getObject());

As a result, a nested pointer (e.g. manager/userName) lost its nesting and
was stored under the leaf name (userName), overwriting the same-named
top-level field. With _fields=userName,manager,manager/userName, the
top-level userName was replaced by the value of manager/userName.

This affected any RequestHandler that does not set the response fields
explicitly (e.g. custom scripted OpenIDM endpoints), because the projection is
performed in InternalConnection via
Resources.filterResource(response, request.getFields()).

Changes

• Resources.filterResource(JsonValue, Collection<JsonPointer>) now uses
  JsonValue.putPermissive(field, value) so each requested JsonPointer is
  written back under its full path, preserving nesting. Missing parent objects
  are created on demand, and same-named leaf fields at different nesting levels
  no longer collide.
  • The empty pointer (field.isEmpty()) still copies all resource fields.
  • Shallow-copy semantics are preserved (value.getObject()).
  • Unresolved pointers (including array-index pointers that do not resolve)
    are skipped, so there is no regression for that case.
• Javadoc updated to document that the projection preserves the nested
  structure of the requested fields and that same-named leaf fields at
  different levels do not conflict.
• Tests in ResourcesTest:
  • Updated testFilterData rows that encoded the old flat behavior to the
    correct nested structure.
  • Added a dedicated test
    (testFilterPreservesNestedFieldsWithCollidingLeafNames) and a data row
    covering the leaf-name collision:
    fields = [userName, manager, manager/userName] over
    { userName: "bjensen", manager: { userName: "jdoe" } }
    must yield result.userName == "bjensen" and
    result.manager.userName == "jdoe".

Behavior

Before:

{ "userName": "jdoe", "manager": { "userName": "jdoe" } }

After:

{ "userName": "bjensen", "manager": { "userName": "jdoe" } }

Testing

mvn -o test -Dtest=ResourcesTest in commons/rest/json-resource — all 106
tests pass.

Related

Once an updated commons 3.1.1-SNAPSHOT is published, the OpenIDM test
openidm-script/.../CustomEndpointFieldProjectionTest will pass.

See OpenIDM discussion:
OpenIdentityPlatform/OpenIDM#183