Dennis/polarsio partition metadata key

[dehume]

Summary & Motivation

Fixes #330. PolarsDeltaIOManager with metadata={"partition_by": ...} regressed in 0.27.12: writing a TimeWindowPartitionsDefinition-partitioned asset failed with Cast error: Cannot cast string '2025' to value of Date32 type.

PR #314 changed get_predicate to emit a DATE '...' literal whenever the asset used a TimeWindowPartitionsDefinition:

if isinstance(partitions_def, TimeWindowPartitionsDefinition):
    return f"DATE '{key}'"

This assumes a time-window partition always maps to a Date column. But the column type is determined by the DataFrame being written, not the partition definition — a %Y yearly partition stored in a string year column produced year = DATE '2025', which deltalake can't cast against a string column.

How I Tested These Changes

• Updated test_partition_filters_predicate to assert the new OR-chain predicate format.
• Added test_polars_delta_native_partitioning_time_window_string_column — end-to-end reproduction of PolarsDeltaIOManager regression in 0.27.12 - partition_by metadata key breaks with Cannot cast string to Date32 #330 (yearly %Y → string column). Verified it fails on the old logic with the exact reported error.
• Added test_polars_delta_native_partitioning_datetime_column_predicate — verifies the overwrite predicate works for a Datetime column. Verified it fails on the old logic with Invalid comparison operation: Timestamp(µs) <= Date32.
• Existing Date-column and multi-partition tests continue to pass.
• Full delta suite passes (21 tests); ruff and ty clean.

Changelog

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