Add unit tests for protobuf encoder/decoder (oxide-sdk)

[niklabh]

Description

The oxide-sdk/src/proto.rs module implements a zero-dependency protobuf wire-format encoder (ProtoEncoder) and decoder (ProtoDecoder). Currently there are zero tests for this module.

Since the protobuf codec is the SDK's native serialisation format and is used by fetch_post_proto, it's important to have thorough test coverage.

Suggested test cases

• Round-trip encoding/decoding for each wire type: uint32, uint64, int32, int64, sint32, sint64, bool, string, bytes, float, double, fixed32, fixed64, sfixed32, sfixed64
• Nested messages — encode a message containing a sub-message, decode and verify
• Empty message — encode with no fields, decode should yield no fields
• Multiple fields — encode several fields, decode all of them in order
• Edge cases — max/min values for integers, empty strings, empty byte arrays, very long strings
• Unknown fields — decoder should gracefully skip unknown field numbers

Files involved

• oxide-sdk/src/proto.rs — add a #[cfg(test)] mod tests { ... } block

Notes

• The SDK compiles to wasm32-unknown-unknown in production, but tests run on the host target. Make sure tests are in a #[cfg(test)] module so they don't affect the WASM build.
• Run tests with: cargo test -p oxide-sdk

Difficulty

Beginner to Intermediate — great way to learn Rust testing patterns and understand the protobuf wire format.