Abilities API: Expand `core/get-user-info` with profile fields and filtering

[gziolo]

Two additive changes to core/get-user-info (shipped in 6.9):

1. Expand the response with five standard profile fields: first_name, last_name, nickname, description, user_url.
2. Add an optional fields input parameter that filters the response to a subset of properties, mirroring the pattern already used by core/get-site-info.

Field naming follows WP-CLI conventions: wp_users columns keep their user_ prefix (user_url); user-meta fields are unprefixed (first_name, last_name, nickname, description).

Proposed shape

Default response (no fields parameter) after the change:

{
  "id": 1,
  "display_name": "Jane Doe",
  "user_nicename": "jane-doe",
  "user_login": "jane",
  "roles": ["administrator"],
  "locale": "en_US",
  "first_name": "Jane",
  "last_name": "Doe",
  "nickname": "Jane",
  "description": "Site administrator and contributor.",
  "user_url": "https://example.com"
}

Filtered response with fields: ["display_name", "first_name", "last_name"]:

{
  "display_name": "Jane Doe",
  "first_name": "Jane",
  "last_name": "Doe"
}

Behavioral rules

• Included by default, opt-out via fields. Omitting fields returns all properties, matching core/get-site-info. Passing fields: ["display_name"] filters down to just that property.
• Schema-validated field names. Each entry in fields must be one of the declared output properties; invalid names are rejected by JSON Schema enum validation.
• Capability check unchanged: is_user_logged_in().
• Annotations unchanged: readonly: true, destructive: false, idempotent: true.

Acceptance criteria

• core/get-user-info returns the five additional fields (first_name, last_name, nickname, description, user_url) alongside the existing 6.9 fields.
• core/get-user-info accepts an optional fields input parameter that filters the response to the requested subset.
• When fields is omitted, the response includes all properties.
• Invalid field names in fields are rejected via schema validation before execution.
• JSON Schema documents each new field with title and description so adapters (MCP, WebMCP) can surface them.
• Unit test coverage in tests/phpunit/tests/abilities-api/ for: expanded read output, fields filtering with valid subset, schema rejection of invalid field names, and unauthenticated denial.