fix: Mark a message as delivered only after it has been fully sent out (#8062)

Fix https://github.com/chatmail/core/issues/8042

The problem was that after receiving the bcc_self'ed pre-message in
`receive_imf`, the logic there only looked for a pending
`smtp`-table-entry that matches the rfc724_mid, and if there was none
then it thought "Great, apparently the message is fully sent out, we can
mark it as delivered!".

But with pre-messages, the same message can have two `smtp` entries (one
for the pre-message and one for the post-message), and the message
should only be marked as delivered once both of them are sent out.

Now, I changed the logic to look for all entries with the same msg_id.
This is actually the same SQL query used in smtp.rs, so, I extracted it
into a new function; feel free to suggest a better name for it.

I tested on Android that it now works fine.

I'll add a test in a follow-up PR.

There are a lot of other problems with sending large files, though:
- The pre-message is sent before the post-message, so that for the
receiver it looks as if the message arrived, but stays in
"downloading..." forever
- There is quite a time delay between clicking on "Send" and the
outgoing message appearing in the chat
- The message shortly gets a letter icon right after it is sent
- I'm wondering if there is a way to give feedback to the user
immediately if the message is too big
- It's unclear when exactly we want to send read receipts

I'll open a follow-up issue for these.

src/test_utils.rs

@@ -40,6 +40,7 @@ use crate::message::{Message, MessageState, MsgId, update_msg_state};
 use crate::mimeparser::{MimeMessage, SystemMessage};
 use crate::receive_imf::receive_imf;
 use crate::securejoin::{get_securejoin_qr, join_securejoin};
+use crate::smtp::msg_has_pending_smtp_job;
 use crate::stock_str::StockStrings;
 use crate::tools::time;
 
@@ -658,10 +659,7 @@ impl TestContext {
             .execute("DELETE FROM smtp WHERE id=?;", (rowid,))
             .await
             .expect("failed to remove job");
-        if !self
-            .ctx
-            .sql
-            .exists("SELECT COUNT(*) FROM smtp WHERE msg_id=?", (msg_id,))
+        if !msg_has_pending_smtp_job(self, msg_id)
             .await
             .expect("Failed to check for more jobs")
         {