Pin structured Snowflake parser in DataFusion 55 fork

[osipovartem]

Summary

• Pin the DF55 fork to Parse nested Snowflake structured types with element nullability on DF55 datafusion-sqlparser-rs#38 so DataFusion and Rustice use one SQL parser revision.
• Keep generic DataFusion planning explicit for the new Snowflake MAP AST; Rustice handles this type through its custom type planner.

Verification

• cargo check --offline -p datafusion-sql
• cargo clippy --offline --all-targets --all-features -- -D warnings
• cargo test --offline -p datafusion-sql --lib (92 passed)
• Full DataFusion suite after initializing test-data submodules: passed
• Independent read-only review of the pin and Rustice integration: approved.

Depends on Embucket/datafusion-sqlparser-rs#38. Rustice integration follows separately.