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/gemini.lua

@@ -91,7 +91,24 @@ function adapter.transform_stream_chunk(raw_chunk)
     local ok, chunk = pcall(json.decode, raw_chunk)
     if not ok then return "" end
 
-    if not chunk.candidates or #chunk.candidates == 0 then return "" end
+    -- Gemini attaches usageMetadata to the final chunk (alongside or after
+    -- candidates). Keys map to Go's TokenUsage json tags (prompt/...).
+    local uses = nil
+    if type(chunk.usageMetadata) == "table" then
+        local p = chunk.usageMetadata.promptTokenCount or 0
+        local c = chunk.usageMetadata.candidatesTokenCount or 0
+        local t = chunk.usageMetadata.totalTokenCount or 0
+        if p > 0 or c > 0 or t > 0 then
+            uses = { prompt = p, completion = c, total = t }
+        end
+    end
+
+    if not chunk.candidates or #chunk.candidates == 0 then
+        if uses ~= nil then
+            return json.encode({ usage = uses, done = false })
+        end
+        return ""
+    end
     local cand = chunk.candidates[1]
     local unified = { content = "", done = (cand.finishReason ~= nil) }
     local reasoning = ""
@@ -117,6 +134,9 @@ function adapter.transform_stream_chunk(raw_chunk)
     end
     if reasoning ~= "" then unified.reasoning_content = reasoning end
     if #tools > 0 then unified.tool_calls = tools end
+    if uses ~= nil then
+        unified.usage = uses
+    end
     return json.encode(unified)
 end