Streaming responses now carry the upstream's real token usage instead of
gateway estimates:
- UnifiedChunk gains an optional Usage field; adapters (opencode, openai)
extract usage from upstream stream chunks (including the final chunk with
empty choices) and pass it through.
- standardSSEChunk preserves usage for passthrough adapters.
- Gateway emits the exact usage in the final stream chunk when available,
falling back to estimates only when the upstream provided none.
Non-streaming usage was already fixed to emit OpenAI-standard keys.
- TokenUsage.MarshalJSON now emits both standard (prompt_tokens,
completion_tokens, total_tokens) and legacy (prompt, completion, total)
keys, so OpenAI-compatible clients (DSH, DevEco Code, etc.) can read
token usage.
- ChatChunk gains an optional Usage field; stream responses now send a
final usage chunk (empty choices) before [DONE].
Refs: usage not visible in clients because the gateway serialized only the
internal short keys and never emitted a streaming usage chunk.
