fix(ios): read MySQL and MariaDB column defaults the way the Mac app does

[datlechin]

Problem

The iOS MySQL driver passed SHOW FULL COLUMNS' Default value through unread. The Mac decoder from #3067 was not shared with the app. So on iPhone and iPad:

• a nullable column's DEFAULT NULL showed no default (Can't set default value of column to Null via query or via Inspector. It remains as empty. #3058's symptom)
• a string default of 'NULL' showed as NULL, and DEFAULT '' showed an empty value
• on MariaDB an expression default and a string default read the same, because its SHOW FULL COLUMNS quotes nothing

Fix

• The iOS app now compiles the Mac decoder instead of a fork of it:
  • MySQLCatalogDefault.swift, MySQLGeneratedColumnClassification.swift, MySQLServerVersion.swift and OceanBaseColumnDefaults.swift are shared.
  • Every top-level declaration in them states nonisolated, and check-ios-shared-isolation.py passes.
• The literal helpers the decoder needs moved from MySQLColumnDefinitionSQL.swift into a Foundation-only MySQLLiteralSQL.swift. The DDL writers stay on the Mac.
• accountSyntax moved into the plugin-only account file, so MySQLServerVersion shares without dragging it in.
• mysqlShowColumnsDefault is the one function the Mac per-table read and the iOS driver both call.
• MySQLColumnListing picks the source by flavor:
  • MySQL and TiDB: SHOW FULL COLUMNS in its bare form
  • MariaDB 10.2.7 and later: the quoted INFORMATION_SCHEMA.COLUMNS default, the only read there that tells an expression from a string. A refused catalog falls back to the bare value, as on the Mac.
  • OceanBase: the shared OceanBaseColumnDefaults.columnDefault
  • Databend: as reported
• The structure view now shows the SQL form ('abc', NULL, CURRENT_TIMESTAMP), the same as the Mac Structure tab.

The iOS app does not read SHOW CREATE TABLE. On MariaDB before 10.2.7, and for a MySQL expression default holding non-ASCII text, a default reads as the catalog states it.

Tests

• MySQLColumnListingTests (9) use SHOW FULL COLUMNS and catalog rows captured verbatim from MySQL 8.4.11 and MariaDB 13.0.2.
• iOS simulator: 777 Swift Testing tests in 105 suites pass, and 53 XCTests with 0 failures (30 are the existing skips).
• SwiftLint is clean on every changed file.
• The Mac suites that cover the moved helpers (MySQLColumnDefinitionSQLTests, OceanBaseColumnDefaultsTests, ColumnDefaultRoundTripTests, MySQLCreateTableTests) did not run locally. The shared build was stuck behind a stalled package download, so CI is their first run.