Moonraker: pass print=true in upload — fixes Upload & Print race with power-on-upload

Closes #14945.

Upload & Print on the Moonraker (Klipper) host type failed with
HTTP 503 "Klippy Host not connected" on any printer that Moonraker
powers up in response to an upload (the [power] on_when_upload_queued
feature). The file landed on disk, the print never started, and the
user hit an error dialog.

Root cause: after POST /server/files/upload succeeds we immediately
fire POST /printer/print/start. On a cold printer that Moonraker just
powered up, Klippy is still coming up when /printer/print/start
arrives, so Moonraker returns 503.

Fix: add `print=true` to the upload multipart form. Moonraker's own
upload endpoint queues the print inside the upload response — and
when a [power] device with on_when_upload_queued is configured, it
powers the printer on and waits for Klippy READY before starting.
That's the whole point of the power-on-upload feature; our second
POST was defeating it.

Also read `result.print_started` from the upload response — when
true, skip our explicit /printer/print/start (Moonraker handled it);
when false (older Moonraker or buddy-fork that ignores the print
flag), fall back to the explicit call so the existing behaviour is
preserved for those servers.

Reporter and root-cause identification: @RubenOllesch.

src/slic3r/Utils/Moonraker.cpp

@@ -212,13 +212,10 @@ bool Moonraker::start_print(wxString &error_msg, const std::string &filename) co
 
 bool Moonraker::upload(PrintHostUpload upload_data, ProgressFn progress_fn, ErrorFn error_fn, InfoFn info_fn) const
 {
-    //ORCA: POST /server/files/upload as multipart/form-data with:
-    //          file = <gcode file>
-    //          root = <storage root>     (Moonraker default: "gcodes")
-    //      Successful response shape:
-    //          { "result": { "item": { "path": "<name>.gcode", "root": "<root>" }, "print_started": <bool> } }
-    //      We always start the print explicitly via /printer/print/start regardless of `print_started`
-    //      so the user can rely on a single call site for state.
+    // POST /server/files/upload with `print=true` so Moonraker queues the print
+    // itself and respects [power] on_when_upload_queued (issue #14945). Older
+    // Moonrakers that ignore the flag return print_started=false and we fall
+    // back to the explicit /printer/print/start below.
     wxString test_msg;
     if (!test(test_msg)) {
         error_fn(std::move(test_msg));
@@ -233,9 +230,12 @@ bool Moonraker::upload(PrintHostUpload upload_data, ProgressFn progress_fn, Erro
     //      addition later (storage picker) needs no change to this method.
     const std::string root = upload_data.storage.empty() ? std::string("gcodes") : upload_data.storage;
 
+    const bool want_start = upload_data.post_action == PrintHostPostUploadAction::StartPrint;
+
     std::string url = make_url("server/files/upload");
     bool result = true;
     std::string uploaded_path;
+    bool moonraker_started_print = false;
 
     //ORCA: gcode inside a .gcode.3mf is index-coded (Metadata/plate_<N>.gcode), so the upload names the
     //      plate via a 1-based `plateindex` (set only in the .3mf path, see Plater::send_gcode_legacy);
@@ -249,13 +249,14 @@ bool Moonraker::upload(PrintHostUpload upload_data, ProgressFn progress_fn, Erro
         % root
         % upload_filename.string()
         % (plateindex.empty() ? "-" : plateindex)
-        % (upload_data.post_action == PrintHostPostUploadAction::StartPrint ? "true" : "false");
+        % (want_start ? "true" : "false");
 
     auto http = Http::post(std::move(url));
     set_auth(http);
     http.form_add("root", root);
     if (!plateindex.empty())
         http.form_add("plateindex", plateindex);
+    http.form_add("print", want_start ? "true" : "false");
     http.form_add_file("file", upload_data.source_path.string(), upload_filename.string())
         .on_complete([&](std::string body, unsigned status) {
             BOOST_LOG_TRIVIAL(debug) << boost::format("%1%: upload HTTP %2%: %3%") % name % status % body;
@@ -278,6 +279,7 @@ bool Moonraker::upload(PrintHostUpload upload_data, ProgressFn progress_fn, Erro
                         "%1%: upload response missing result.item.path, falling back to original filename `%2%`")
                         % name % uploaded_path;
                 }
+                moonraker_started_print = ptree.get<bool>("result.print_started", false);
             } catch (const std::exception &ex) {
                 BOOST_LOG_TRIVIAL(warning) << boost::format(
                     "%1%: could not parse upload response (%2%); falling back to original filename")
@@ -306,7 +308,8 @@ bool Moonraker::upload(PrintHostUpload upload_data, ProgressFn progress_fn, Erro
     if (!result)
         return false;
 
-    if (upload_data.post_action == PrintHostPostUploadAction::StartPrint && !uploaded_path.empty()) {
+    // Fallback when Moonraker ignored the `print` flag or reported print_started=false.
+    if (want_start && !uploaded_path.empty() && !moonraker_started_print) {
         wxString start_msg;
         if (!start_print(start_msg, uploaded_path)) {
             error_fn(std::move(start_msg));