fix(bigquery): preserve reported types in unit test fixtures

[fornwall]

Unit-test fixtures become SQL rows whose types must match the referenced columns. The renderer previously worked from Arrow data types alone: BigQuery GEOGRAPHY and JSON both look like strings. Fixture casts also reused the type mapping for dbt seed, which chooses DATETIME for timestamp values even when the referenced column is TIMESTAMP.

For example, this model returns GEOGRAPHY, TIMESTAMP, and JSON columns:

-- models/fixture_types.sql
SELECT
  ST_GEOGPOINT(100, -37) AS location,
  TIMESTAMP '2026-01-01 00:00:00+00' AS created_at,
  PARSE_JSON('{"segmentCode":"HORECA"}') AS payload;

A unit test supplies the matching expected row:

# models/fixture_types.yml
unit_tests:
  - name: preserves_column_types
    model: fixture_types
    given: []
    expect:
      rows:
        - location: "ST_GEOGPOINT(100, -37)"
          created_at: "2026-01-01 00:00:00+00"
          payload: '{"segmentCode":"HORECA"}'

Before this fix, the expected fixture row used these casts (simplified SQL):

SELECT
  CAST('ST_GEOGPOINT(100, -37)' AS STRING) AS location,
  CAST('2026-01-01 00:00:00+00' AS DATETIME) AS created_at,
  CAST('{"segmentCode":"HORECA"}' AS STRING) AS payload;

The STRING values are incompatible with the model's GEOGRAPHY and JSON columns. The timestamp cast fails because its UTC offset is invalid for DATETIME.

With this fix, the same fixture uses:

SELECT
  CAST(ST_GEOGPOINT(100, -37) AS GEOGRAPHY) AS location,
  CAST('2026-01-01 00:00:00+00' AS TIMESTAMP) AS created_at,
  CAST(PARSE_JSON('{"segmentCode":"HORECA"}') AS JSON) AS payload;

The renderer preserves the reported BigQuery type from field metadata, including nested arrays and structs and NUMERIC/BIGNUMERIC values. Empty arrays get explicit element types. JSON and GEOGRAPHY are excluded from ORDER BY, which BigQuery does not support for these types. The fix reuses the existing type lookup and rendering paths; other adapters keep their current mappings.

Related: #13804, #14271, #14625, #15708, #15882