SNOW-3556267: Fix AttributeError when write_pandas receives list name…

[JonathanStefanov]

1. Which Jira issue is this PR addressing? Make sure that there is an accompanying issue to your PR.
   Fixes SNOW-3556267

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.

When Session.write_pandas is called with database= and schema= kwargs, _write_modin_pandas_helper builds a list like ["MY_DB", "MY_SCHEMA", "MY_TABLE"] and passes it as name to to_snowflake. The below-threshold path re-routes to the Snowflake backend which unpacks that list correctly. The Parquet path passed the whole list to a function that called .upper() on it.

write_pandas already accepts table_name, schema, and database as separate string arguments. So the #fix splits the list before calling it: last element is always the table name, second-to-last is the schema (if present), first is the database (if 3 parts). Each part is individually converted from Snowflake identifier format to the plain string write_pandas expects.

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[JonathanStefanov]

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