Parse nested Snowflake structured types with element nullability on DF55

[osipovartem]

Summary

• Backport the Snowflake ARRAY/MAP nested type and NOT NULL AST support from Snowflake: Parse structured OBJECT types apache/datafusion-sqlparser-rs#2489 to the DF55-compatible Embucket parser fork.
• Preserve formatting and parsing round trips for nested structured types.

Verification

• cargo test --offline --all-features
• cargo clippy --offline --all-targets --all-features -- -D warnings
• cargo fmt --check
• Independent read-only review: approved.

The Rustice integration and DataFusion parser pin are separate dependent PRs.