Add VARIANT appender shredding test

[wmTJc9IK0Q]

What

Adds TestQueryAppenderVariantShredding (appender_test.go), a test that bulk-inserts
semi-structured key-value data with keys not known at compile time into a
VARIANT column through the appender API, and proves the DuckDB storage engine
shreds the values.

Why

The direct data-chunk appender and prepared-statement binds both reject VARIANT
(TestAppenderRejectsVariant, appender.go, statement.go): Go vector setters can't
emit VARIANT and there's no C-API helper to construct a variant. This test documents
and locks in the one path that does work and verifies it end to end, including the storage-level shredding behavior introduced in DuckDB 1.5.

How it works

• Insertion: NewQueryAppender with a temp table holding an INTEGER id and a
  VARCHAR JSON document; the INSERT casts col2::JSON::VARIANT, producing a proper
  OBJECT(x, y) variant. Keys come from runtime data (map[string]any → json.Marshal),
  not a compile-time struct, so arbitrary/unknown keys are supported.
• Proof of shredding (A/B): on an on-disk database attached with
  STORAGE_VERSION 'v1.5.0', identical rows are appended into two tables that differ only
  by variant_minimum_shredding_size (-1 disabled vs 0 forced) at their respective
  CHECKPOINT — the setting is GLOBAL and consulted at checkpoint time.
  pragma_storage_info then shows typed value segments present only in the shredded
  table (zero in the unshredded baseline), which is direct storage-engine evidence.
• Round-trip: confirms variant_typeof(col) = OBJECT(x, y) and col.x/col.y extract
  the original values.

Notes

• Relies on the json extension bundled with the duckdb-go bindings (already used by
  examples/json and TestJSON); a SELECT '{}'::JSON warmup loads it before the
  appender INSERT.
• Integer field values are used deliberately: they yield a distinct typed value segment
  that is provably absent without shredding, giving an unambiguous signal.

[taniabogatsch]

Thanks, looks good to me! Could you just have a look at the failing linter?