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/provider/provider_test.go

@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"path/filepath"
+	"strings"
 	"sync/atomic"
 	"testing"
 	"time"
@@ -472,3 +473,14 @@ func TestImageAutoUsesImageModel(t *testing.T) {
 		t.Fatalf("AUTO image must use the image-kind model, got %q", gotModel)
 	}
 }
+
+func TestStandardSSEChunkFinishReason(t *testing.T) {
+	out := standardSSEChunk(`{"choices":[{"index":0,"finish_reason":"tool_calls","delta":{}}]}`)
+	if !strings.Contains(out, `"done":true`) || !strings.Contains(out, `"finish_reason":"tool_calls"`) {
+		t.Fatalf("real finish reason must pass through: %s", out)
+	}
+	out = standardSSEChunk(`{"choices":[{"index":0,"finish_reason":"","delta":{"content":"hi"}}]}`)
+	if strings.Contains(out, `"Done":true`) {
+		t.Fatalf("empty-string finish reason is not a finish signal: %s", out)
+	}
+}