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

Fixes SNOW-NNNNNNN

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

Please describe how your code solves the related issue.

When SQL simplifier is disabled, Snowpark builds synthetic schema queries via schema_expression(). For non-null DecFloatType columns, that path used DECFLOAT '0', which Snowflake describes as nullable. That caused DecFloat arithmetic result columns (for example NEW_E in test_plus_basic) to be reported as nullable=True.

This change uses 0 :: DECFLOAT for non-null DecFloat schema placeholders instead, matching other numeric types and preserving NOT NULL metadata in describe queries.