Fix GREATEST with single argument simplified in Databricks SQL output

[moomindani]

Summary

Fixes #2308

Spark SQL requires GREATEST to have more than one argument:

The `greatest` requires > 1 parameters but the actual number is 1.

Oracle (and other dialects) allow GREATEST(single_expr) as a redundant no-op wrapper. This PR adds a custom _greatest_sql transform in lakebridge's Databricks generator that:

• Single argument: unwraps GREATEST(expr) → expr
• Multiple arguments: keeps the normal GREATEST(a, b, ...) behaviour unchanged

Example

Input (Oracle)	Output before	Output after
GREATEST(NVL(col, TO_DATE('1000-01-01', 'YYYY-MM-DD')))	GREATEST(COALESCE(col, TO_DATE('1000-01-01'))) ❌	COALESCE(col, TO_DATE('1000-01-01')) ✅
GREATEST(col1)	GREATEST(col1) ❌	col1 ✅
GREATEST(a, b)	GREATEST(a, b) ✅	GREATEST(a, b) ✅

Changes

• src/databricks/labs/lakebridge/transpiler/sqlglot/generator/databricks.py: Add _greatest_sql function and register it in TRANSFORMS
• tests/resources/functional/oracle/functions/test_greatest_1.sql: Multi-arg GREATEST (no change)
• tests/resources/functional/oracle/functions/test_greatest_2.sql: Single-arg GREATEST with NVL → simplifies
• tests/resources/functional/oracle/functions/test_greatest_3.sql: Single-arg GREATEST with column → simplifies

Test plan

• tests/unit/transpiler/test_oracle.py — all 4 cases pass (including 3 new)
• tests/unit/transpiler/test_snowflake.py::test_snowflake[test_greatest_1] — multi-arg case unchanged

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.43%. Comparing base (7ff77c2) to head (661a60d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2328      +/-   ##
==========================================
+ Coverage   66.41%   66.43%   +0.01%     
==========================================
  Files          99       99              
  Lines        9094     9098       +4     
  Branches      974      975       +1     
==========================================
+ Hits         6040     6044       +4     
  Misses       2878     2878              
  Partials      176      176              

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[asnare]

I'm afraid this PR doesn't address #2308 as intended: the reported issue reports a problem in the Switch transpiler/component, and that's not part of this repository.

The files that have been modified are part of a legacy transpiler that is not in use; it's currently dead code and scheduled for removal.