* Stop Malformed Network Responses from Crashing the App
Duet, MKS and UltiMaker parsed print host replies with boost read_json
inside the HTTP completion callback with no try, so an HTML or truncated
reply threw out of the Physical Printer Test button and terminated the app,
or killed the upload queue thread. The five identical copies of the parser
(ESP3D's and Flashforge's were unused) are replaced by one shared
PrintHost::get_err_code_from_body that reports a non-JSON reply as an error.
The upload queue now catches a failing job per job, so one bad upload no
longer leaves later jobs queued forever.
Flashforge read material station slots with nlohmann value(), which throws
on off-type fields or non-object entries. The parsing moves into
Flashforge::parse_material_slots, which reads fields leniently with the
existing try_parse_json_int and skips bad entries.
UserManager::parse_json parsed the payload before its try block; the parse
now happens inside it.
* Keep UploadFinished Paired with UploadStarted When an Upload Throws
The exception from a throwing upload was caught around perform_job, so
the UploadFinished lifecycle event was skipped and plugins saw an
upload start that never finished.
The catch now sits around the upload call. The error is reported
through the job's error callback and UploadFinished is fired with an
error code, as for any other failed upload. The worker keeps running
for the next job. The started, upload and finished sequence moved to
PrintHostJobQueue::upload_job so it can be tested without the dialog.
