SNOW-2360274: Fix schema query sql generation for structured types

[sfc-gh-jrose]

1. Which Jira issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

   Fixes SNOW-2360274

2. Fill out the following pre-review checklist:

   • I am adding a new automated test(s) to verify correctness of my new code
     • If this test skips Local Testing mode, I'm requesting review from @snowflakedb/local-testing
   • I am adding new logging messages
   • I am adding a new telemetry message
   • I am adding new credentials
   • I am adding a new dependency
   • If this is a new feature/behavior, I'm adding the Local Testing parity changes.
   • I acknowledge that I have ensured my changes to be thread-safe. Follow the link for more information: Thread-safe Developer Guidelines
   • If adding any arguments to public Snowpark APIs or creating new public Snowpark APIs, I acknowledge that I have ensured my changes include AST support. Follow the link for more information: AST Support Guidelines

3. Please describe how your code solves the related issue.

The sql generation for describe queries has an edge case that can cause invalid sql to be generated when using nested structured types. The schema_expression function does it's best to respect nullability when generating schema queries. Snowflake does not support nullability inside of structured types so this should not be needed for structured types. This fix short circuits the sql generation by replacing the recursive call with NULLs. I have not found any cases in which this would break, but to be extra sure it's not a BCR I am gating it behind a context variable. In order to use this bugfix you will have to set context. _enable_fix_2360274 to be True.