fix: If importing a backup fails, delete the partially-imported profile (#7885)

fix https://github.com/chatmail/core/issues/7863

`test_import_encrypted_bak_into_encrypted_acct` CFFI test fails because
it tests that trying to import an encrypted account with a wrong
passphrase into an already-encrypted database will fail, but leave the
already-encrypted database (esp, leave it with the same password).

But in order to reset the database after a failed login attempt, I'm
using this code:

```rust
        context.sql.close().await;
        fs::remove_file(context.sql.dbfile.as_path())
            .await
            .log_err(context)
            .ok();
        context
            .sql
            .open(context, "".to_string()) // <-- NOTE THIS LINE
            .await
            .log_err(context)
            .ok();
```

We're not remembering the password, so, we can't just pass the correct
password there.

Since password-protected databases are not really supported anyways, we
decided to just skip the test.

I also tried two tricks for deleting everything [found on
Stackoverflow](https://stackoverflow.com/questions/525512/drop-all-tables-command),
but neither of them managed to actually reset the database (i.e. they
led to a failed Rust test, because asserting
`!context2.is_configured().await?` failed):

```rust
        context
            .sql
            .call_write(|conn| {
                let mut stmt = conn.prepare(
                    "select 'drop table ' || name || ';' from sqlite_master where type = 'table';",
                )?;
                let mut iter = stmt.query(())?;
                while iter.next()?.is_some() {}
                Ok(())
            })
            .await
            .log_err(context)
            .ok();
        context
            .sql
            .run_migrations(context)
            .await
            .log_err(context)
            .ok();
```

```rust
        context
            .sql
            .transaction(|t| {
                t.execute_batch(
                    "
PRAGMA writable_schema = 1;
delete from sqlite_master where type in ('table', 'index', 'trigger');
PRAGMA writable_schema = 0",
                )?;
                Ok(())
            })
            .await
            .log_err(context)
            .ok();
        context
            .sql
            .run_migrations(context)
            .await
            .log_err(context)
            .ok();
```

---------

Co-authored-by: l <link2xt@testrun.org>

python/tests/test_3_offline.py

@@ -558,6 +558,12 @@ class TestOfflineChat:
         assert messages[0 + E2EE_INFO_MSGS].text == "msg1"
         assert os.path.exists(messages[1 + E2EE_INFO_MSGS].filename)
 
+    @pytest.mark.skip(
+        reason="We didn't find a way to correctly reset an account after a failed import attempt "
+        "while simultaneously making sure "
+        "that the password of an encrypted account survives a failed import attempt. "
+        "Since passphrases are not really supported anymore, we decided to just disable the test.",
+    )
     def test_import_encrypted_bak_into_encrypted_acct(self, acfactory, tmp_path):
         """
         Test that account passphrase isn't lost if backup failed to be imported.