Slack: add modal private metadata utilities

src/slack/modal-metadata.ts

@@ -0,0 +1,42 @@
+export type SlackModalPrivateMetadata = {
+  sessionKey?: string;
+  channelId?: string;
+  channelType?: string;
+};
+
+const SLACK_PRIVATE_METADATA_MAX = 3000;
+
+function normalizeString(value: unknown) {
+  return typeof value === "string" && value.trim().length > 0 ? value.trim() : undefined;
+}
+
+export function parseSlackModalPrivateMetadata(raw: unknown): SlackModalPrivateMetadata {
+  if (typeof raw !== "string" || raw.trim().length === 0) {
+    return {};
+  }
+  try {
+    const parsed = JSON.parse(raw) as Record<string, unknown>;
+    return {
+      sessionKey: normalizeString(parsed.sessionKey),
+      channelId: normalizeString(parsed.channelId),
+      channelType: normalizeString(parsed.channelType),
+    };
+  } catch {
+    return {};
+  }
+}
+
+export function encodeSlackModalPrivateMetadata(input: SlackModalPrivateMetadata): string {
+  const payload: SlackModalPrivateMetadata = {
+    ...(input.sessionKey ? { sessionKey: input.sessionKey } : {}),
+    ...(input.channelId ? { channelId: input.channelId } : {}),
+    ...(input.channelType ? { channelType: input.channelType } : {}),
+  };
+  const encoded = JSON.stringify(payload);
+  if (encoded.length > SLACK_PRIVATE_METADATA_MAX) {
+    throw new Error(
+      `Slack modal private_metadata cannot exceed ${SLACK_PRIVATE_METADATA_MAX} chars`,
+    );
+  }
+  return encoded;
+}