feat(gateway): pass through exact upstream token usage in streams

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.

internal/types/types.go

@@ -138,6 +138,11 @@ type UnifiedChunk struct {
 	Done             bool            `json:"done"`
 	ToolCalls        json.RawMessage `json:"tool_calls,omitempty"`
 	ReasoningContent string          `json:"reasoning_content,omitempty"`
+	// Usage carries the upstream token usage when the stream chunk provides
+	// it (OpenAI-style streams attach usage to some chunks; the final one
+	// often has empty choices). Gateway uses it to emit exact usage in the
+	// final stream chunk instead of estimates.
+	Usage *TokenUsage `json:"usage,omitempty"`
 }
 
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