refactor(barretenberg-rs): migrate to ipc-codegen + ipc-runtime; drop PipeBackend

[charlielye]

Summary

Migrates barretenberg-rs to the generated bb API client from #23612 and the shared /ipc-runtime Rust bindings from #23610.

This PR sits between the bb server migration and the bb.js migration.

Stack

1. feat(ipc): add /ipc-codegen and /ipc-runtime foundation packages #23610 cl/ipc-foundation
2. refactor(wsdb): migrate aztec-wsdb to ipc-codegen + ipc-runtime #23611 cl/ipc-wsdb-migrate
3. refactor(bbapi): migrate bb binary to ipc-codegen + ipc-runtime #23612 cl/ipc-bb-migrate
4. refactor(barretenberg-rs): migrate to ipc-codegen + ipc-runtime; drop PipeBackend #23613 cl/ipc-bb-rs-migrate — this PR
5. refactor(bb.js): migrate to ipc-codegen + ipc-runtime; delete legacy bb::ipc #23614 cl/ipc-bb-js-migrate
6. refactor: cut-over to new wsdb package - comms over uds ipc. #23036 cl/ipc-3-avm-wsdb-cutover
7. feat: add generated aztec-vm-sim package setup #23084 cl/ipc-4-avm-binary
8. feat: cut simulator over to generated bb-avm-sim IPC service #23697 cl/ipc-5-avm-cutover

What changes

• Generates Rust bb types and client code from bb_schema.json.
• Uses the generated BbApi over ipc_runtime::IpcClient.
• Removes the old hand-written Rust backend/types/error layers.
• Removes the legacy PipeBackend; Rust clients now use the IPC runtime path.
• Keeps the FFI entrypoint conventional and non-configurable.
• Updates tests for fixed-size bin32/alias handling from the schema.

Validation

• barretenberg-rs release build with local BB_LIB_DIR.
• Rust tests adjusted for generated fixed-size values and conventional FFI entrypoint.