Snowflake: parse nested COPY transformation paths

[osipovartem]

Which issue does this PR close?

Closes no issue.

Rationale for this change

Snowflake permits nested semi-structured paths in staged COPY transformations, for example $1:country:name::VARCHAR. The dedicated simple staged-field parser currently consumes $1:country and leaves :name::VARCHAR behind, so the documented statement fails to parse.

Documentation: https://docs.snowflake.com/en/user-guide/tutorials/script-data-load-transform-parquet

What changes are included in this PR?

• Fall back to the standard expression parser when a staged field is followed by another path operator, bracket access, qualification, or cast.
• Extend the existing COPY cast-transformation test with the documented nested-path shape.

Simple staged fields keep their compact StageLoadSelectItem AST; only complex expressions use the existing JsonAccess / Cast AST.

Are there any user-facing changes?

Yes. Snowflake COPY statements with nested staged paths now parse and round-trip correctly.

How was this change tested?

• cargo test --test sqlparser_snowflake (164 passed)
• cargo clippy --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check