[FIX] Avoid 500 upon Mail repository download

[chibenwa]

java.io.IOException: written 579 > 418 content-length
tat org.eclipse.jetty.server.internal.HttpChannelState$ChannelResponse.write(HttpChannelState.java:1381)

[quantranhong1999]

Failed
org.apache.james.webadmin.routes.MailRepositoriesRoutesTest.downloadingAMailShouldReturnTheEml
1 expectation failed.
Expected status code <200> but was <500>.

Standard Output
Request method: GET
Request URI: http://localhost:37933/mailRepositories/myRepo/mails/name1
Proxy:
Request params:
Query params:
Form params:
Path params:
Headers: Accept=message/rfc822
Content-Type=application/json
Cookies:
Multiparts:
Body:

HTTP/1.1 500 Server Error
Server: Jetty(12.1.7)
Date: Mon, 15 Jun 2026 15:12:19 GMT
Cache-Control: must-revalidate,no-cache,no-store
Content-Length: 0