Preserve historical manifest bounds after schema evolution

[osipovartem]

Summary

• Preserve historical manifest-list partition bounds when copying entries into a new commit after schema promotion or partition-source removal.
• Keep snapshot scans on the scanned snapshot schema, including the bounds-with-nulls pruning path.
• Restore historical decoding for the default public Avro entry helper; callers that need scan-schema decoding can use the explicit schema variant.

Validation

• Regression: v3 manifest copy after INT -> LONG promotion and dropped partition source, plus mixed INT/LONG snapshot bounds for pruning.
• cargo +1.99.0 test -p iceberg-rust-spec --lib: 195 active tests passed.
• cargo +1.99.0 test -p iceberg-rust --lib table::manifest_list: 7 passed.
• cargo +1.99.0 test -p iceberg-rust --lib table::transaction: 16 active tests passed.
• cargo +1.94.0 clippy -p iceberg-rust -p iceberg-rust-spec --all-targets -- -D warnings: passed.
• Independent read-only review: APPROVED after fixing both identified blockers.

Rust 1.99 strict Clippy reports pre-existing useless_borrows_in_formatting lints in untouched spec files; this PR was validated with the upstream PR's Rust 1.94 toolchain.