Allow data definitions to declare example values

Problem/Motivation

Data definitions describe valid values, but can’t include examples. That leaves API docs without a concrete value to show and forms without useful placeholder text. generateSampleValue() creates random test data, so it doesn’t fill this need.

Steps to reproduce

Not applicable (feature request).

Proposed resolution

Add optional examples metadata to DataDefinition, with setExamples() and getExamples(). Docs and forms can use these examples. They’re descriptive only: they won’t be validated or affect runtime behavior.

Remaining tasks

• Follow-up: emit examples from the JSON Schema normalizers. (See #3623432: Emit data definition example values as the JSON Schema "examples" keyword)

User interface changes

None.

Introduced terminology

None.

API changes

Two new methods on DataDefinition (and potentially DataDefinitionInterface, subject to the same base-class-first BC approach as other metadata additions). Purely additive.

Data model changes

None. The definition array gains an optional key.

Release notes snippet

Data definitions can now declare example values via DataDefinition::setExamples(). Examples are descriptive metadata for consumers such as JSON Schema emitters, generated API documentation, and form placeholders; they are not validated and do not affect runtime behavior.

AI usage (if applicable)

• [x] AI Assisted Issue: This issue was generated with AI assistance, but was reviewed and refined by the creator.
• [x] AI Assisted Code: This code was mainly generated by a human, with AI autocompleting or parts AI generated, but under full human supervision.