fix(MailTransmission): Handle clients/servers picky about empty headers

[joshtrichards]

Came up on the forum:

• https://help.nextcloud.com/t/emails-sent-from-nextcloud-contain-an-empty-cc-field/240195
• using nextcloud as an email client -> Email sent to foobar@baz.com contains non reverse-alias addresses simple-login/app#2676

Changes:

• New function buildHeaders to have same logic applied to sendMessage, saveDraft, saveLocalDraft
• Set Cc and Bcc header on demand
• Drop the detour through ModelMessage
• Drop Date header (done by Horde Mime later)

[kesselb]

Thanks a lot @joshtrichards for flagging it 👍

I've noticed that while working on #11043 #11889, and forgot to follow up 🙈
Indeed, we should not pass empty headers; that's just weird.

The CC/BCC headers are Horde_Mail_Rfc822_Address objects and do not evaluate to empty.
I'm going to push a fixup to address it.

[Copilot]

Pull request overview

This pull request fixes an issue where some email clients and servers reject messages containing empty Cc or Bcc headers. The fix refactors the header building logic to only include Cc and Bcc headers when they actually contain recipients.

Changes:

• Introduced a new buildHeaders() method that conditionally adds Cc and Bcc headers only when they contain recipients
• Removed the unnecessary detour through ModelMessage class
• Removed the Date header from draft operations (handled automatically by Horde_Mime_Mail)

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
lib/Service/MailTransmission.php	Added buildHeaders() method and refactored sendMessage(), saveLocalDraft(), and saveDraft() to use it; removed ModelMessage usage and Date header from drafts
tests/Unit/Service/MailTransmissionTest.php	Added two new tests to verify Cc headers are included when present and omitted when empty

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