fix(python): handle missing manifest partition summaries

[mattfaltyn]

Which issue does this PR close?

• Closes Python ManifestFile.partitions panics when partition summaries are absent #2883.

What changes are included in this PR?

The Python ManifestFile.partitions getter now returns None when the optional core partition summaries are absent. Previously, the getter unconditionally unwrapped the optional value and triggered a Rust/PyO3 panic for valid manifest entries with partitions: None.

The optional return type preserves the distinction between absent summaries and a present empty list, avoids cloning the entire summaries vector before converting its entries, and a focused regression test covers the absent-summary case.

Are these changes tested?

Yes. The regression test constructs a valid ManifestFile with partitions: None and verifies that the getter returns None.

• cargo test --locked -p pyiceberg_core_rust test_manifest_partitions_without_summaries -- --nocapture
• cargo fmt --all -- --check
• cargo clippy --locked -p pyiceberg_core_rust --all-targets --all-features -- -D warnings

[github-actions]

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[dannycjones]

Thanks for the PR! I think we should consider changing the return type here