fix(channels): allow config field in channel instance updates

Add "config" to channelInstanceAllowedFields so channel config changes
from the UI are actually persisted to the database instead of being
silently dropped by the allowlist filter.

internal/http/validate.go

@@ -65,5 +65,5 @@ var mcpServerAllowedFields = map[string]bool{
 var channelInstanceAllowedFields = map[string]bool{
 	"channel_type": true, "credentials": true, "agent_id": true,
 	"enabled": true, "group_policy": true, "allow_from": true,
-	"metadata": true, "webhook_secret": true,
+	"metadata": true, "webhook_secret": true, "config": true,
 }