feat: add ws chat attachments

src/gateway/chat-attachments.ts

@@ -0,0 +1,51 @@
+export type ChatAttachment = {
+  type?: string;
+  mimeType?: string;
+  fileName?: string;
+  content?: unknown;
+};
+
+export function buildMessageWithAttachments(
+  message: string,
+  attachments: ChatAttachment[] | undefined,
+  opts?: { maxBytes?: number },
+): string {
+  const maxBytes = opts?.maxBytes ?? 2_000_000; // 2 MB
+  if (!attachments || attachments.length === 0) return message;
+
+  const blocks: string[] = [];
+
+  for (const [idx, att] of attachments.entries()) {
+    if (!att) continue;
+    const mime = att.mimeType ?? "";
+    const content = att.content;
+    const label = att.fileName || att.type || `attachment-${idx + 1}`;
+
+    if (typeof content !== "string") {
+      throw new Error(`attachment ${label}: content must be base64 string`);
+    }
+    if (!mime.startsWith("image/")) {
+      throw new Error(`attachment ${label}: only image/* supported`);
+    }
+
+    let sizeBytes = 0;
+    try {
+      sizeBytes = Buffer.from(content, "base64").byteLength;
+    } catch {
+      throw new Error(`attachment ${label}: invalid base64 content`);
+    }
+    if (sizeBytes <= 0 || sizeBytes > maxBytes) {
+      throw new Error(
+        `attachment ${label}: exceeds size limit (${sizeBytes} > ${maxBytes} bytes)`,
+      );
+    }
+
+    const safeLabel = label.replace(/\s+/g, "_");
+    const dataUrl = `![${safeLabel}](data:${mime};base64,${content})`;
+    blocks.push(dataUrl);
+  }
+
+  if (blocks.length === 0) return message;
+  const separator = message.trim().length > 0 ? "\n\n" : "";
+  return `${message}${separator}${blocks.join("\n\n")}`;
+}