feat(azure-sql): Transparent Data Encryption (TDE)

[NitinKumar004]

What

Adds Azure SQL Transparent Data Encryption (the Microsoft.Sql/servers/databases/transparentDataEncryption/current sub-resource) so real armsql.TransparentDataEncryptionsClient code works against the emulator.

• New optional TransparentDataEncryptions capability on the relationaldb driver (Set/Get/List), discovered by type assertion — the cross-cloud driver is not widened.
• A TDE record is auto-materialized as Enabled when a database is created, matching Azure's encrypted-at-rest default, so a Get/List round-trips with no explicit PUT.
• Wire handler branches on rp.SubResourceAction == "transparentDataEncryption" inside serveDatabaseRoute; the trailing /current name segment is safely dropped by the 4-segment ParsePath, and Get vs. ListByDatabase is disambiguated on the raw path suffix.

Architecture-fit

• Follows the existing sql sub-resource pattern (firewallRules / elasticPools): a self-contained server/azure/sql/tde.go handler + a providers/azure/sql/tde.go backend on a new memstore.Store keyed "server/database".
• Sync-200, no LRO, no Delete — real Azure SQL TDE PUT is synchronous (the SDK method is CreateOrUpdate, not Begin…; it accepts 200/201/202 and does not poll). A 202 would be wrong; TDE also cannot be deleted, only toggled.
• No new ARG triple: TDE is a settings sub-resource of an existing database, not a discoverable ARM resource type, so it needs no Resource Graph row.

Persistence

The TDE store bolts onto the already-Snapshottable sql Mock, which the completeness guard does not deep-check — so it was added manually to the snapshot dumps/loads lists and the sqlSnapshot struct, and the round-trip is asserted in snapshot_test.go. Cascade cleanup is wired: DeleteDatabase drops the record, and the server-delete prefix cascade (deleteChildren) covers it, so an RG purge leaves no orphans.

Test

• Real-SDK e2e (server/azure/sql/tde_sdk_test.go): a freshly-created DB reports Enabled with no PUT; CreateOrUpdate(state=Enabled) → Get round-trips; ListByDatabase returns the single current record.
• Provider snapshot round-trip covers the auto-materialized TDE record.

Refs #611.