Allow data definitions to declare a default value

Problem/Motivation

Typed data definitions describe a value’s type and constraints, but can’t specify a default. Other parts of Drupal handle defaults separately, so tools built from a data definition can’t easily use one.

Proposed resolution

Add an optional default to DataDefinition, with setDefault() and getDefault() methods. It’s descriptive metadata only; it won’t change how values are created or validated. The method names should avoid clashing with the existing field API.

Remaining tasks

• Follow-up issues: emit default from the JSON Schema normalizers (See #3623431: Emit data definition default values as the JSON Schema "default" keyword); evaluate delegating ContextDefinition::getDefaultValue() to the underlying definition.

User interface changes

None.

Introduced terminology

None.

API changes

Two new methods on DataDefinition (and potentially DataDefinitionInterface, subject to the BC discussion above). Purely additive; no existing method changes signature or behavior.

Data model changes

None. The definition array gains an optional key.

Release notes snippet

Data definitions can now declare a default value via DataDefinition::setDefault(). The default is descriptive metadata available to consumers such as JSON Schema emitters and form generators; it does not change how typed data is instantiated or validated.

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.