add 25 MB per-file attachment size guard to MessageFormatter

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Packages/MagnumOpusCore/Sources/SMTPClient/MessageFormatter.swift

@@ -71,10 +71,21 @@ public enum MessageFormatter: Sendable {
 	}
 
 	/// Format a multipart/mixed message with attachments.
+	/// Throws `MessageFormatterError.attachmentTooLarge` if any file exceeds 25 MB.
 	public static func formatMultipart(
 		_ message: OutgoingMessage,
 		attachments: [(filename: String, mimeType: String, data: Data)]
-	) -> String {
+	) throws -> String {
+		let maxSize = 25 * 1024 * 1024 // 25 MB
+		for attachment in attachments {
+			if attachment.data.count > maxSize {
+				throw MessageFormatterError.attachmentTooLarge(
+					filename: attachment.filename,
+					size: attachment.data.count
+				)
+			}
+		}
+
 		let boundary = "=_MagnumOpus_\(UUID().uuidString)"
 
 		var headers: [(String, String)] = []
@@ -184,3 +195,7 @@ public enum MessageFormatter: Sendable {
 		return result
 	}
 }
+
+public enum MessageFormatterError: Error {
+	case attachmentTooLarge(filename: String, size: Int)
+}