[Divyansh151005]

Fixes #702.

What this does

MessageSet extensions carry their type id as an int32, so protoc allows extension numbers up to i32::MAX on messages with message_set_wire_format = true. buffa previously capped every extension at (1 << 29) - 1 in both DescriptorPool::link_extension and codegen for extend, so descriptors protoc accepts were rejected or failed to generate.

This change passes a per-call upper bound into link_field: i32::MAX when the extendee is a MessageSet, otherwise MAX_FIELD_NUMBER. Codegen uses the same rule via validated_extension_field_number, resolving the extendee message in the input descriptor set.

Testing

Rust 1.95.0 (rust-toolchain.toml).

cargo fmt --all --check
cargo test -p buffa-codegen message_set_extension_const
cargo test -p buffa-descriptor --test message_set_extension_field_number --features reflect
cargo clippy -p buffa-codegen -p buffa-descriptor --features buffa-descriptor/reflect -- -D warnings

Results: both unit tests passed (message_set_extension_const_uses_large_field_number, message_set_extension_above_max_field_number_links); fmt and clippy completed with no warnings.

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}