Fix padded type_ids in Python encode paths

[grp06]

Summary

• Route Python tokenizer padding through PyEncoding::pad so padded type_ids use the configured pad_type_id.
• Remove the duplicated ids/mask-only padding helper.
• Add right and left padding unit coverage for the shared encoding builder.

Tests

• cargo test -p fastokens-python encoding_pad_applies_pad_type_id
• cargo test -p fastokens-python encode_batch_pad_type_id_applied_to_type_ids
• cargo test -p fastokens-python build_encoding_left_padding_applies_pad_type_id
• cargo test -p fastokens-python --lib
• cargo fmt --check
• cargo clippy -- -D warnings
• cargo test

[AlonKejzman]

Hello, thank you for the PR! Will look at it soon.