fix(duckdb): partition initial DuckLake writes

[anxkhn]

Description

Fixes #5742.

For partitioned DuckLake tables created from a query, create the table schema without writing data, apply the partition specification, and then insert the initial rows. This ensures the first DuckLake data files are physically partitioned, including for unannotated SQL models and fully qualified DuckLake targets.

Test Plan

• uv run make style
• uv run pytest -o addopts='' tests/core/engine_adapter/test_duckdb.py -q (13 passed)
• uv run make fast-test (2582 passed, 4 skipped; 6 unrelated MSSQL/Fabric tests failed because local pyodbc cannot load the missing unixODBC library)

Checklist

• I have run make style and fixed any issues
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test)
• My commits are signed off (git commit -s) per the DCO