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/gateway/chat.go

@@ -580,6 +580,32 @@ func (g *Gateway) writeRec(rec *Req) {
 	g.stats.Record(*rec)
 }
 
+// mergeUsage combines token usage across stream chunks additively. Some
+// providers split usage across chunks (e.g. Anthropic reports prompt tokens
+// in message_start and the final completion tokens in message_delta); a plain
+// "last non-nil wins" would discard the prompt half. Non-zero fields from cur
+// override prev; total is recomputed from the merged parts so a partial later
+// chunk can't shrink it. For the common single-chunk case (OpenAI's terminal
+// empty-choices+usage chunk) upstream totals are preserved exactly.
+func mergeUsage(prev, cur *types.TokenUsage) *types.TokenUsage {
+	if prev == nil {
+		u := *cur
+		if u.Total == 0 && (u.Prompt > 0 || u.Completion > 0) {
+			u.Total = u.Prompt + u.Completion
+		}
+		return &u
+	}
+	out := *prev
+	if cur.Prompt > 0 {
+		out.Prompt = cur.Prompt
+	}
+	if cur.Completion > 0 {
+		out.Completion = cur.Completion
+	}
+	out.Total = out.Prompt + out.Completion
+	return &out
+}
+
 func (g *Gateway) streamChat(w http.ResponseWriter, ctx context.Context, cands []*provider.Provider, req *types.ChatRequest, effective string, rec *Req) {
 	rec.LatMs = 0
 	t0 := time.Now()
@@ -632,7 +658,7 @@ func (g *Gateway) streamChat(w http.ResponseWriter, ctx context.Context, cands [
 	var lastUsage *types.TokenUsage
 	for ck := range chunks {
 		if ck.Usage != nil {
-			lastUsage = ck.Usage
+			lastUsage = mergeUsage(lastUsage, ck.Usage)
 		}
 		chunk := ChatChunk{
 			ID: id, Object: "chat.completion.chunk", Created: created, Model: effective,
@@ -802,7 +828,7 @@ func (g *Gateway) streamChatAuto(w http.ResponseWriter, ctx context.Context, cha
 	var lastUsage *types.TokenUsage
 	for ck := range chunks {
 		if ck.Usage != nil {
-			lastUsage = ck.Usage
+			lastUsage = mergeUsage(lastUsage, ck.Usage)
 		}
 		chunk := ChatChunk{
 			ID: id, Object: "chat.completion.chunk", Created: created, Model: rec.Model,