feat(gateway): pass through upstream finish_reason end-to-end

The gateway hardcoded "stop" on every terminating stream chunk, so
tool-call rounds reported finish_reason=stop and length caps were
invisible to clients. UnifiedChunk now carries finish_reason; adapters
emit it (with empty-string finish reasons like sensenova treated as
non-terminal), standardSSEChunk passes it through for un-adapted
upstreams, [DONE] no longer emits a duplicate reason-less done chunk,
and both streaming paths emit the real reason with "stop" as fallback.

Also vendor sensenova/agentrouter adapters into the repo: they were
WebUI-only uploads and a deploy sync silently removed them while live
AUTO-chain slots still referenced them.

internal/lua/adapters/ollama.lua

@@ -85,16 +85,27 @@ function adapter.transform_stream_chunk(raw_chunk)
         end
     end
 
+    -- Ollama done_reason -> OpenAI finish_reason ("length" 透传，其余归一 stop)
+    local finish = nil
+    if chunk.done then
+        if chunk.done_reason == "length" then
+            finish = "length"
+        else
+            finish = "stop"
+        end
+    end
+
     if not chunk.message then
         if uses ~= nil then
-            return json.encode({ content = "", done = true, usage = uses })
+            return json.encode({ content = "", done = true, finish_reason = finish, usage = uses })
         end
-        return ""
+        return json.encode({ content = "", done = true, finish_reason = finish })
     end
 
     local unified = {
         content = chunk.message.content or "",
-        done = chunk.done or false
+        done = chunk.done or false,
+        finish_reason = finish
     }
     if uses ~= nil then
         unified.usage = uses