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/anthropic.lua

@@ -99,9 +99,37 @@ end
 function adapter.transform_stream_chunk(raw_chunk)
     local ok, chunk = pcall(json.decode, raw_chunk)
     if not ok then return "" end
-    if chunk.type == "message_start" then return "" end
+    if chunk.type == "message_start" then
+        local uses = nil
+        if chunk.message and type(chunk.message.usage) == "table" then
+            local u = chunk.message.usage
+            local p = u.input_tokens or 0
+            local c = u.output_tokens or 0
+            if p > 0 or c > 0 then
+                uses = { prompt = p, completion = c, total = p + c }
+            end
+        end
+        if uses ~= nil then
+            return json.encode({ usage = uses, done = false })
+        end
+        return ""
+    end
     if chunk.type == "message_delta" then
-        return json.encode({ content = "", done = (chunk.delta and chunk.delta.stop_reason ~= nil) })
+        local uses = nil
+        if type(chunk.usage) == "table" then
+            local u = chunk.usage
+            local p = u.input_tokens or 0
+            local c = u.output_tokens or 0
+            if p > 0 or c > 0 then
+                uses = { prompt = p, completion = c, total = p + c }
+            end
+        end
+        local done = (chunk.delta and chunk.delta.stop_reason ~= nil)
+        if uses ~= nil then
+            -- completion is final here; prompt is merged from message_start
+            return json.encode({ content = "", done = done, usage = uses })
+        end
+        return json.encode({ content = "", done = done })
     end
     if chunk.type == "content_block_start" and chunk.content_block
         and chunk.content_block.type == "tool_use" then