feat(python): Support partitioned tables in sink_iceberg

[ldacey]

• add _resolve_partition_key to map pyicberg partition transforms to polars expressions for PartitionBy

This extends the work done here: #26799

I have been using sink_parquet with PartitionBy along with pyiceberg add_files in order to get lazy streaming writes to my Iceberg tables. Comparing my code with the committed code above I only had to make a few changes:

• remove NotImplementedError for partitioned tables
• mapping the Iceberg partition transforms to Polars expressions to pass to PartitionBy

Bucket transforms are not supported due to using murmur3 hashing.

I tested this with the simple pytests and a 6 million row table with various partitioning (Month(date), Day(date), Identity(location)) directly to my GCS catalog and to local iceberg tables.

1. I used AI to generate pytests similar to the existing sink_iceberg tests and to compare feat: Add unstable LazyFrame.sink_iceberg #26799 versus my local code to understand the scope (to make sure I would not need to touch Rust code etc)
2. I confirm that I have reviewed all changes myself, and I believe they are
   relevant and correct.

@nameexhaustion

[codecov]

Codecov Report

❌ Patch coverage is 91.17647% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.64%. Comparing base (9d60226) to head (4a04089).
⚠️ Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/src/polars/io/iceberg/_sink.py	91.17%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26864      +/-   ##
==========================================
+ Coverage   81.00%   81.64%   +0.64%     
==========================================
  Files        1805     1805              
  Lines      248021   248052      +31     
  Branches     3132     3140       +8     
==========================================
+ Hits       200902   202533    +1631     
+ Misses      46313    44713    -1600     
  Partials      806      806              

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[ldacey]

Is there an issue with the code or a better approach?

[nameexhaustion]

We're just not ready for this yet