[datlechin]

Summary

• Cassandra and ScyllaDB list a keyspace's user-defined types under Types, read from system_schema.types. Opening one shows its fields and a rebuilt CREATE TYPE, with Open in Editor.
• The structure editor's type picker offers the table's keyspace types. An entry is wrapped in frozen<…> when a plain column cannot hold it: a field is a non-frozen collection, the server predates 3.6, or it is ScyllaDB (reports 3.0.8). Names are quoted like Cassandra's maybeQuote, plus CQL type names, so a type called "text" is never spelled as the native text.
• Drop Type… on a type row, for every engine whose driver writes the statement: Cassandra and ScyllaDB, PostgreSQL and PGlite (DROP DOMAIN for a domain), SQL Server. The gate shows the statement and confirms it as destructive; the server's refusal while a table or type still uses it is shown as is. Hidden on read-only connections.
• PluginKit, additive (kit stays 40): PluginUserDefinedTypeKind.udt ("User-Defined Type", CQL's word, as feat(plugin-mssql): browse SQL Server alias, table and CLR types #2856 did for SQL Server) and generateDropTypeSQL(type:) defaulting to nil. MCP list_types accepts kind: "udt"; UserDefinedTypeToolSchemaTests is updated for it.
• CassandraObjectQueries.elements(of:) no longer trims spaces and quotes off list elements, which renamed a field or argument such as "customer'".
• Cancelling the gate's confirmation of Refresh Materialized View or Edit Comment no longer reports "Operation cancelled by user" as a failure.
• apple/scripts/probes/check-cassandra-user-types.sh runs the type code against a live server.
• Changelog: 3719.added.md, 3719.added.1.md, 3719.fixed.md, 3719.fixed.1.md.

Test plan

• UserTypeDroppingTests, new: the drop runs on the scope's pooled driver, a denial runs nothing, a cancel stays a cancel, the adapter forwards the type, Support lists droppable kinds.
• DatabaseTreeMenuSpecTests, UserDefinedTypeInfoTests, PluginMetadataRegistryCuratedCapabilityTests (Cassandra and ScyllaDB browse types before the plugin loads), PostgreSQLTypeQueryTests: fail on main, pass here. 190 tests in the touched suites pass; AllPlugins builds.
• Probe against Cassandra 5.0.9: every rebuilt CREATE TYPE round-trips, including names with quotes, spaces and commas; each picker spelling is accepted and is the type ("text" and "byte" included); the bare name is refused wherever the spelling is frozen; DROP TYPE is refused while in use and runs once nothing uses the type. Not run against ScyllaDB or Cassandra 3.x/4.x.

Not in this PR

• A SQL dump writes types by name, so a type whose field uses a later-named type (contact holding frozen<phone>, or a PostgreSQL composite over a later domain) is created before its dependency and the restore stops there (SQLExportPlugin.writeObjectCreatePhase, sorted by (dumpOrder, name)).
• Materialized views still do not appear in the Cassandra sidebar.

Docs: TableProApp/docs#76

Fixes #2528