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 an empty list 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 getter also 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 Python getter returns an empty list.

• cargo test -p pyiceberg_core_rust test_manifest_partitions_without_summaries -- --nocapture
• make check
• make unit-test
• (cd bindings/python && make test) — 17 passed, 2 skipped