Convert tagged ARRAY values for Variant-backed INSERT targets

[osipovartem]

Summary

• Convert logical ARRAY inputs to native Variant storage during INSERT planning only when the source is tagged as ARRAY and the target is Variant-backed.
• Keep same-storage INSERT and ordinary string inputs on their existing paths.
• Add positive and negative planner tests.

This is a one-commit follow-up to the DataFusion revision currently pinned by Rustice. The Rustice integration uses this commit together with a provenance rule that traces the inserted TO_VARIANT wrapper back to its ARRAY source.

Validation

• cargo +stable check --offline -p datafusion-sql --test sql_integration
• cargo +stable test --offline -p datafusion-sql --test sql_integration plan_insert_ -- --nocapture (7 passed)
• cargo +stable fmt --all -- --check
• Independent read-only review: no blocking findings.

The full Rustice executor test is pending the workspace disk-space threshold; this PR makes no claim of end-to-end validation yet.