feat(variant): allow creating VariantArray from Option items convertible to Variant

[jabrailkhalil]

Rationale

Closes #8628.

Previously VariantArray could be collected only from Option<Variant> items:

let arr: VariantArray = values.iter().map(|v| Some(Variant::from(*v))).collect();

With this change, any type that converts cleanly into Variant can be collected directly:

let arr: VariantArray = vec![Some(1_i64), None, Some(42_i64)].into_iter().collect();

let arr: VariantArray = vec![Some("hello"), Some("a very long string exceeding the short string limit")].into_iter().collect();

This follows the suggestion made in #8625 (comment).

Changes

• parquet-variant-compute/src/variant_array.rs: generalize FromIterator<Option<Variant<'m, 'v>>> to impl<'m, 'v, V: Into<Variant<'m, 'v>>> FromIterator<Option<V>>, converting each item through Into::into. Existing code that collects Option<Variant> items keeps working (identity Into).
• Added regression tests: i64 items with nulls, &str items covering both short-string (ShortString) and long-string (String) storage.

Verification

• cargo test -p parquet-variant-compute --lib — 366 tests passed (364 existing + 2 new), including the new test_from_option_into_variants_into_variant_array and test_from_option_str_into_variant_array.
• cargo clippy -p parquet-variant-compute --all-targets -- -D warnings — clean.
• cargo fmt --all -- --check — clean.

Note: local verification used the x86_64-pc-windows-gnu target; upstream CI covers the full matrix.

AI disclosure

Per the Arrow project's AI-generated submissions guidance: this change was implemented with the assistance of an AI coding agent. The author reviewed the final diff, verified it compiles and passes tests/clippy/fmt locally, and takes full ownership of the change.

The ASF ICLA may need to be on file before merge (asf-clabot will report the status).