feat(protocol): add revision 2 multimodal contract and packages

[connorcarpenter15]

Summary

• advance the OpenEngine schema to revision 2 while retaining minimum client revision 1 compatibility
• add structured multimodal capability discovery and per-request media options for image, video, and audio
• preserve the existing multimodal compatibility field and extend generation without removing revision 1 fields
• add reproducible Rust and Python openengine-proto 0.2.0 package generation, generated-code drift checks, release checks, and cross-language fixtures

Why

The Dynamo TRT-LLM sidecar needs one engine-neutral contract for aggregate generation, context-first prefill/decode handoff, multimodal inputs, LoRA lifecycle, health, load, drain, cancellation, and KV-event discovery. Keeping this contract in OpenEngine lets both consumers pin the same immutable schema release.

Compatibility

Servers advertise schema revision 2 and a minimum client revision of 1. Existing supports_multimodal behavior remains available while newer clients can use the typed modality/source capability fields.

Validation

• Buf build and lint
• generated-code drift and descriptor checks
• Rust binding tests: 4 passed
• installed Python wheel tests: 6 passed
• Python package-content and Twine checks
• Python-to-Rust and Rust-to-Python fixture round trips, including 64-bit identifiers and binary handoff state