feat(sqlserver): implement insert_record with IDENTITY_INSERT handling (#147)

[saurabh500]

Summary

Implements Driver::insert_record for the SQL Server driver with proper IDENTITY column handling. Closes #147.

Changes

• sqlserver/helpers.rs — new pure helper build_insert_sql(qualified, columns, wrap_identity_insert) that emits either a plain INSERT … VALUES (@P1, @P2, …) or, when an IDENTITY column is being supplied, wraps the insert in BEGIN TRY … BEGIN TRAN; SET IDENTITY_INSERT … ON; INSERT …; SET IDENTITY_INSERT … OFF; COMMIT TRAN; END TRY BEGIN CATCH IF @@TRANCOUNT > 0 ROLLBACK TRAN; SET IDENTITY_INSERT … OFF; THROW; END CATCH. IDENTITY_INSERT is session-scoped and not reverted by rollback, so the CATCH block explicitly turns it back OFF before re-raising.
• sqlserver/helpers.rs — new helper value_to_sql_param(&Value) -> Box<dyn ToSql> that dispatches on the JSON variant to the bridge's typed primitive ToSql impls (bool, i64, f64, String, Option::<String>::None). The bridge's blanket impl ToSql for serde_json::Value JSON-stringifies every variant into NVARCHAR(4000) (filed upstream), which silently breaks inserts into bit/int/float/etc columns.
• sqlserver/introspection.rs — new detect_identity_column(conn, schema, table) async fn + Q_GET_IDENTITY_COLUMN constant. Queries sys.columns filtered by is_identity = 1 for the resolved object_id.
• sqlserver/mod.rs — replaces the read-only stub with a real implementation:
  1. Validate non-empty data
  2. detect_identity_column to learn the table's IDENTITY column (if any)
  3. Sort columns deterministically and check whether the user-provided data includes the IDENTITY column → derive needs_identity_insert
  4. build_insert_sql with the right wrapping
  5. Bind params via value_to_sql_param, execute, return execute_result.total()

Tests

• 9 new unit tests in helpers.rs:
  • 4 covering build_insert_sql (plain INSERT, identifier escaping, identity wrapping shape, custom target name)
  • 1 covering value_to_sql_param dispatch across Bool/Number(i64)/Number(f64)/String/Null/Array/Object
• 1 new unit test in introspection.rs covering Q_GET_IDENTITY_COLUMN query shape
• Updated write_operations_error_out_in_phase1 — insert_record no longer errors out with "read-only" since the stub is gone; now asserts the empty-data validation message. Delete + create_view still assert "read-only".

All 102 driver tests pass (cargo test --lib drivers::sqlserver).

Bridge limitation note

preview.3's execute() returns 0 for DML row counts due to an mssql-tds DONE token limitation (bridge issue #1). Callers should verify via SELECT, not the return value. This PR returns .total() as-is — accurate row count handling is tracked by the bridge upgrade work (PR for #145).

Out of scope (followups)

• Testcontainers-gated integration tests covering plain INSERT, identity-provided INSERT, and CATCH-path rollback.
• Removing manifest readonly: true once update/delete are also implemented.

Closes #147