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/lua/adapters/openai.lua

@@ -67,7 +67,24 @@ function adapter.transform_stream_chunk(raw_chunk)
     local ok, chunk = pcall(json.decode, raw_chunk)
     if not ok then return "" end
 
-    if not chunk.choices or #chunk.choices == 0 then return "" end
+    -- OpenAI-style streams may attach usage to a chunk with empty choices
+    -- (the final usage chunk). Keys must match Go's TokenUsage json tags
+    -- (prompt/completion/total); the gateway re-emits standard *_tokens.
+    local uses = nil
+    if type(chunk.usage) == "table" then
+        uses = {
+            prompt = chunk.usage.prompt_tokens or chunk.usage.prompt or 0,
+            completion = chunk.usage.completion_tokens or chunk.usage.completion or 0,
+            total = chunk.usage.total_tokens or chunk.usage.total or 0,
+        }
+    end
+
+    if not chunk.choices or #chunk.choices == 0 then
+        if uses ~= nil then
+            return json.encode({ usage = uses, done = (chunk.usage ~= nil) })
+        end
+        return ""
+    end
     local delta = chunk.choices[1].delta or {}
     local fr = chunk.choices[1].finish_reason
 
@@ -75,11 +92,13 @@ function adapter.transform_stream_chunk(raw_chunk)
         content = delta.content or "",
         done = (fr ~= nil)
     }
+    if uses ~= nil then
+        unified.usage = uses
+    end
     if delta.reasoning_content then
         unified.reasoning_content = delta.reasoning_content
     end
     if delta.tool_calls then
-        -- pass raw streaming fragments through; OpenAI clients accumulate index+id+name+arguments
         unified.tool_calls = delta.tool_calls
     end
     return json.encode(unified)