feat(storage/sql): one file per SQL provider instead of switches in sql.go

[Lutherwaves]

Check Existing Issues

• I have searched the existing issues and discussions.

Problem Description

Each SQL provider's specifics are spread across switch statements in storage/sql.go. Adding a provider means editing all of them, plus a new Lucene dialect file:

Function	Provider-specific part
OpenConnection	building the connection (DSN)
CreateSchema	skipping schema creation on SQLite
CreateMigrationTable	a different table definition for each provider
UpdateMigrationTable	SQLite vs. everything else
GetLatestMigration	SQLite vs. everything else

The default branches make it easy to cover a provider by accident. For example, CreateMigrationTable creates migrations on MySQL, while UpdateMigrationTable and GetLatestMigration use <schema>.migrations. Not verified against MySQL yet.

Desired Solution you'd like

Use the same pattern as storage/search/lucene/dialect.go: one private interface, one file per provider, one lookup table.

type sqlProvider interface {
	dialector(config map[string]string) (gorm.Dialector, error)
	migrationsTable(schema string) string
	migrationsDDL(schema string) string
	hasSchemas() bool
}

var sqlProviders = map[StorageProviders]sqlProvider{
	POSTGRESQL: postgresProvider{},
	MYSQL:      mysqlProvider{},
	SQLITE:     sqliteProvider{},
}

• sql_postgres.go, sql_mysql.go, sql_sqlite.go hold the provider code. postgresDSN / mysqlDSN from fix(storage/sql): escape connection string values #265 move there as-is.
• SQLAdapter looks up its provider once in OpenConnection, and the switches become method calls.
• The "supported providers" error is built from the map keys.
• Adding a provider means adding one file and one map entry.

Keep it private: an exported registry would be new API that nobody needs yet. Use an explicit map, not init() registration.

Additional Context

• Should be a pure refactor with no behavior change. Land it after fix(storage/sql)!: scope Update to the filtered row #258 and feat(storage)!: typed options, and let Update write only the named fields #262, which both touch sql.go.
• Check the MySQL migrations table mismatch above separately, before this refactor.
• Came up while reviewing fix(storage/sql): escape connection string values #265.