Summary

Adds support for the PostgreSQL citext extension type to the migrations adapter.

The heavy lifting was already in place: Cake\Database\Schema\PostgresSchemaDialect::columnDefinitionSql() already emits CITEXT for TableSchemaInterface::TYPE_CITEXT, and schema reflection already handles it (see line 147 of that dialect). The only thing blocking the type from reaching the dialect was the migrations adapter itself — Column::$type passes through to isValidColumnType() via in_array($column->getType(), $this->getColumnTypes(), true), and PostgresAdapter::$specificColumnTypes did not include citext, so builder calls like

$table->addColumn('nickname', 'citext', ['null' => true])

failed with

An invalid column type "citext" was specified for column "nickname".

Changes, modelled on how cidr / inet / macaddr are wired:

• Expose AdapterInterface::TYPE_CITEXT (mirrors TableSchemaInterface::TYPE_CITEXT)
• Add Column::CITEXT alias
• Include self::TYPE_CITEXT in PostgresAdapter::$specificColumnTypes
• Add a Postgres integration test (testAddColumnCitext) — note that the test bootstrap already runs CREATE EXTENSION IF NOT EXISTS citext, which strongly suggests this support was intended
• Add TYPE_CITEXT (expected false) to the SqliteAdapter provideColumnTypesForValidation data provider
• Document the new type in writing-migrations.md, noting that the user still needs to enable the extension on the database (the migration will not create it automatically)

No changes to DDL generation are needed — that already works end-to-end through the Cake dialect.

Notes

• A follow-up to this would be a generic hook for registering arbitrary Postgres extension types (hstore, ltree, vector, tsvector, …). That is a larger design discussion and deliberately not included here — this PR narrowly fixes citext, which Cake core already supports.