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/kimicode.lua

@@ -119,10 +119,15 @@ function adapter.transform_stream_chunk(raw_chunk)
     local delta = chunk.choices[1].delta or {}
     local fr = chunk.choices[1].finish_reason
 
+    local finish = (type(fr) == "string" and fr ~= "") and fr or nil
+
     local unified = {
         content = delta.content or "",
-        done = (fr ~= nil)
+        done = (finish ~= nil)
     }
+    if finish then
+        unified.finish_reason = finish
+    end
     if uses ~= nil then
         unified.usage = uses
     end