fix(gateway): pass through upstream token usage in streams for all adapters

The prior usage-passthrough fix only covered openai/opencode; the same
empty-choices+usage drop bug remained in the 5 sibling OpenAI-compatible
adapters, and non-OpenAI providers (anthropic/gemini/ollama) never surfaced
streaming usage at all.

- deepseek/github/groq/kimicode/mistral: preserve usage on empty-choices
  chunks and attach it to normal chunks (same pattern as openai.lua)
- anthropic: emit usage from message_start (prompt) and message_delta
  (completion); gateway merges split usage additively
- gemini: read usageMetadata in the stream path
- ollama: fix non-streaming key (usage -> token_usage, matches
  UnifiedResponse json tag) and read prompt_eval_count/eval_count;
  surface counts from the done stream chunk
- gateway: mergeUsage combines usage across chunks (non-zero fields win,
  total recomputed from prompt+completion) so split usage doesn't lose
  the prompt half; single-chunk case (OpenAI) preserved exactly
- usage-only chunks: done=false (no redundant terminal stop), matching
  the Go fallback standardSSEChunk

internal/lua/adapters/github.lua

@@ -63,7 +63,27 @@ end
 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
+            -- usage-only chunk is not a content/finish signal; the gateway
+            -- emits its own terminal stop chunk and merges this usage.
+            return json.encode({ usage = uses, done = false })
+        end
+        return ""
+    end
     local delta = chunk.choices[1].delta or {}
     local fr = chunk.choices[1].finish_reason
 
@@ -71,6 +91,9 @@ 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