fix: mediaUrl fallback to directPath for newsletter uploads

Newsletter endpoints return only direct_path (no url field).
Use direct_path as fallback so mediaUrl is never silently undefined.
Also update WAMediaUploadFunction return type to reflect that
mediaUrl may be undefined when only direct_path is present.

Resolves Copilot review comments on PR #311.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/Types/Message.ts

@@ -992,7 +992,7 @@ export type WAMediaUploadFunction = (
 	encFilePath: string,
 	opts: { fileEncSha256B64: string; mediaType: MediaType; timeoutMs?: number; newsletter?: boolean }
 ) => Promise<{
-	mediaUrl: string
+	mediaUrl: string | undefined
 	directPath: string
 	meta_hmac?: string
 	ts?: number

src/Utils/messages-media.ts

@@ -892,7 +892,7 @@ export const getWAUploadToServer = (
 
 				if (result?.url || result?.direct_path) {
 					urls = {
-						mediaUrl: result.url!,
+						mediaUrl: result.url || result.direct_path!,
 						directPath: result.direct_path!,
 						meta_hmac: result.meta_hmac,
 						fbid: result.fbid,