feat(sqlserver): Part 6 — SQL type mapping and column builder

[axellpadilla]

Part 6 of the SQL Server Fusion adapter series. Part of dbt-labs#15714.

Stacked on #15 (Part 5). Base is part-5-sqlserver-catalog, so this diff shows only Part 6's two files. Merge #11 → #12 → #13 → #14 → #15 first; this retargets automatically as each lands.

sql_types.rs and column_builder.rs — the two files rg '\bFabric\b' flags as Part 6's work list. Adds SqlServer everywhere Fabric already has an arm, matching Fabric's value where SQL Server's T-SQL surface agrees and diverging where it measurably doesn't.

Where it diverges from Fabric

• STRING → VARCHAR(MAX), not Fabric's VARCHAR(8000). Fabric Warehouse tracks its own byte cap in FABRIC_MAX_VARCHAR_TYPE; on-prem/Azure SQL Database has no equivalent limit tracked here. VARCHAR(MAX) is v1's current native mapping (dbt_sqlserver_use_native_string_types, default True as of 1.12) — the legacy VARCHAR(8000) path is deprecated in v1 and isn't what this ports.
• Decimal → "float"/"int" by scale, not Fabric's unconditional "float". v1's convert_number_type is "float" if decimals else "int"; the new SqlServer arms in the scale-keyed match reproduce that threshold instead of Fabric's flat mapping.
• Boolean ("bit"), timestamp ("datetime2(6)"), and time ("time(6)") match both v1 and Fabric — no divergence, just added.

Column builder

build_fabric, not build_postgres_like, is the right model here — confirmed by reading rather than assumed: build_postgres_like hardcodes Column::new(AdapterType::Postgres, ...) (would mislabel every SQL Server column) and special-cases Timestamp/Time64 to a bare "datetime" literal regardless of adapter (would lose the datetime2(6) precision v1 relies on). build_sqlserver mirrors build_fabric's shape instead, backed by the new sqlserver::try_format_type module.

Left alone

• get_field_sql_type_metadata_key gets todo!() for SqlServer, matching the other adapters that already work and are already todo!() there (Databricks, Postgres, Salesforce, Spark, DuckDB, Alt). The function is unused outside its own Bigquery-only unit test; the metadata key that's actually live is dbt_adapter_sql::types::SQLSERVER_KEYS.
• adapter_impl.rs's remaining SqlServer E0004s are Part 7's scope, not this file pair's — a full cargo check -p dbt-adapter still fails until feat(sqlserver): Part 7 — adapter_impl.rs match arms #7 lands, same as it did before this PR.

Verification

cargo check -p dbt-adapter --tests reports zero errors in sql_types.rs or column_builder.rs (all remaining errors are in adapter_impl.rs, Part 7's file). Full cargo test can't run yet — the crate doesn't build until Part 7 closes the gap — so the new unit tests (sqlserver_try_format_type_formats_native_types, sqlserver_try_format_type_rejects_unsupported_arrow_types, plus SqlServer assertions added to the existing convert_*_type tests) are type-checked but not yet executed.

PRs against sqlserver-v2-port in this repo, use Closes #<this issue>.

Closes #6