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/anthropic.lua

@@ -124,12 +124,23 @@ function adapter.transform_stream_chunk(raw_chunk)
                 uses = { prompt = p, completion = c, total = p + c }
             end
         end
-        local done = (chunk.delta and chunk.delta.stop_reason ~= nil)
+        local finish = nil
+        if chunk.delta and chunk.delta.stop_reason ~= nil then
+            -- Anthropic stop_reason -> OpenAI finish_reason
+            local sr = chunk.delta.stop_reason
+            if sr == "max_tokens" then
+                finish = "length"
+            elseif sr == "tool_use" then
+                finish = "tool_calls"
+            else
+                finish = "stop"
+            end
+        end
         if uses ~= nil then
             -- completion is final here; prompt is merged from message_start
-            return json.encode({ content = "", done = done, usage = uses })
+            return json.encode({ content = "", done = (finish ~= nil), finish_reason = finish, usage = uses })
         end
-        return json.encode({ content = "", done = done })
+        return json.encode({ content = "", done = (finish ~= nil), finish_reason = finish })
     end
     if chunk.type == "content_block_start" and chunk.content_block
         and chunk.content_block.type == "tool_use" then