feat(api): add engine-specific extra extension field for smoother adoption

[tanmayv25]

Summary

Adds a capability-optional, engine-specific extension field (google.protobuf.Struct extra) to the request and discovery surfaces, plus one field-number hygiene fix. All changes are purely additive — verified wire-compatible against main.

The goal is to lower adoption friction: an engine can expose OpenEngine (and surface a native knob or native discovery metadata) without waiting for the spec to grow a typed field for it. extra is explicitly outside the portable contract — an engine may ignore keys it doesn't recognize, every request stays valid with it empty, and clients never depend on it for correctness. Parameters that prove broadly useful get promoted to typed fields in a later revision.

Changes

Location	Field	Purpose
GenerateRequest	extra = 12	native generation / sampler knobs
TaskRequestContext	extra = 4	native embed / classify / score knobs
ServerInfo	extra = 12	native server metadata (attention backend, build flags)
ModelInfo	extra = 28	native model metadata (kv-cache dtype, quantization)
KvOptions	reserved 2;	retire the pre-existing tag gap so it can never be silently reused

docs/api.md updated to match, with a prose note documenting the extra policy.

Why these four locations

They are the surfaces where heterogeneous engines diverge — two request roots and two discovery responses. The other open-ended surfaces already have an escape hatch (EngineError.details, KvSessionRef.attributes_struct as Struct; LoadInfo/RuntimeEvent via attributes maps), so no new field was added there. Per-request sub-messages (SamplingParams, *Options) and per-output messages were intentionally left typed to avoid a stringly-typed sprawl.

Compatibility & verification

• protoc compile: OK
• buf lint (STANDARD): PASS
• buf breaking vs main: PASS — no wire-incompatible changes

🤖 Generated with Claude Code