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/types/types.go

@@ -136,6 +136,10 @@ type ImageGenResponse struct {
 type UnifiedChunk struct {
 	Content          string          `json:"content"`
 	Done             bool            `json:"done"`
+	// FinishReason carries the upstream finish/stop reason ("tool_calls",
+	// "length", ...) when the adapter provides it; the gateway emits it on
+	// the terminating chunk instead of the default "stop".
+	FinishReason     string          `json:"finish_reason,omitempty"`
 	ToolCalls        json.RawMessage `json:"tool_calls,omitempty"`
 	ReasoningContent string          `json:"reasoning_content,omitempty"`
 	// Usage carries the upstream token usage when the stream chunk provides