fix(openai): align image streaming relay governance

Route OpenAI image streaming through shared stream handling, split image/realtime/usage helpers for maintainability, and include the related image request and rate limit updates.

relay/helper/stream_scanner.go

@@ -22,8 +22,8 @@ import (
 )
 
 const (
-	InitialScannerBufferSize    = 64 << 10 // 64KB (64*1024)
-	DefaultMaxScannerBufferSize = 64 << 20 // 64MB (64*1024*1024) default SSE buffer size
+	InitialScannerBufferSize    = 64 << 10  // 64KB (64*1024)
+	DefaultMaxScannerBufferSize = 128 << 20 // 64MB (64*1024*1024) default SSE buffer size
 	DefaultPingInterval         = 10 * time.Second
 )