feat: Assess Microsoft SQL Server schema impact without a database connection - #23466

Deviations from the PostgreSQL assessor (#23463), and why:

• Source-only column changes are reported as NoChange columns. SQL Server maps both list<string> and json to nvarchar(max), so Datadog v5.19.10→v6.0.0 datadog_monitors.tags is a no-op for migrate. Without this, the finding would have no columns and would not show the SQL Server types. The table is No destination schema change, with tags: nvarchar(max) → nvarchar(max), "makes no changes".
• normalizeTable clears Unique. The plugin never creates unique constraints, so a table read back from SQL Server never has one. Without this, a removed source unique constraint would report Manual migration required, but migrate makes no change.
• No rule extraction. unsafeChanges/needsTableDrop (client/changes.go) and normalizeTable (client/schema.go) are already package-level and pure, so the assessor calls them directly.
• Close returns early when db is nil, because a no_connection client has no DB or writer.

Verification:

• make test passed (client and queries packages, against SQL Server 2019 in Docker).
• golangci-lint run --config ../../.golangci.yml: no issues.
• I broke the Unique normalization on purpose. TestAssessTablesRemovedUniqueConstraint failed, and passed again after I restored it.
• I ran a throwaway live test (not committed) against the same Docker SQL Server. It compared each assessment to a real safe-mode MigrateTables:
  • list<string> → json: NoChange, and migrate succeeded.
  • int → bigint: ManualMigrationRequired, and migrate rejected it.
  • Added nullable column: AutomaticallyMigratable, and migrate succeeded.

plugin-sdk is pinned to the unreleased ath-938-add-the-destination-assessment-interface-and-server-wiring branch (cloudquery/plugin-sdk#2618). ATH-985 replaces it with a release.