Rebuild the open tab a pack overwrites

[saworbit]

Fixes #1072

Problem

scene_pack_branch with overwrite: true saves inside the editor with ResourceSaver.save. A tab that had the target open kept its old tree, and the next editor_save_scene wrote it back over the pack. The pack said saved: true and the save said saved.

What changed

• scene_pack_branch now runs the same editor copy refresh as the file writers after a pack that saved. The cached copy is reloaded and an open tab is rebuilt from the packed file. The answer carries editor_copy_reloaded and, when a tab was rebuilt, editor_scene_reloaded.
• overwrite: true is the consent to lose the tab's unsaved changes, the same call viewport_create_test_lab and scene_create make. So the pack never refuses over an open tab, and there is no new argument.
• On 4.5 and 4.6 a rebuilt tab left of a current last tab stayed the edited scene, and later scene calls acted on it. This hit the [Editor sync] Reload an open scene tab when a file writer rewrites its scene #1068 writers too. The bridge now answers edited_scene_moved_from, and the server opens that scene again with scene.open once the tabs are done.
• Along the way: scene_create over a scene no tab holds no longer asks for a reload. On 4.7 that printed Can't reload scene ..., as it's not opened. The new harness case caught it.
• Along the way: the harness allow entry for the deliberately broken fixture script also matches Failed parse script, the wording 4.5.1 prints when a GDScript language server client connects and the editor re-parses every script.

Engine facts this rests on

Read from editor/editor_node.cpp at 4.5.1, 4.6.2 and 4.7.2, and measured with a live editor on each:

• 4.5 and 4.6 end reload_scene with scene_tabs->set_current_tab(current_tab). The tab bar already sits at that index when the rebuilt tab was left of a current last tab, so nothing switches back. 4.7 rewrote the recovery and keeps the current tab.
• open_scene_from_path and reload_scene_from_path return early while is_changing_scene() is set, which lasts until the deferred _set_main_scene_state at the end of the frame. So the switch back has to be a later request.
• 4.7's reload_scene fails with an error for a path no tab holds. 4.5 and 4.6 clear the current scene's undo history instead.

Tests

• Native: a pack rebuilds the target's tab after the pack and never before it, with discard_unsaved. A refused pack asks nothing. A rebuild that moves the edited scene is switched back once, to the scene the caller was on. A switch back the editor refuses is named with the way out. A rebuild that keeps the current scene asks nothing more.
• Live harness, Invoke-PackBranchTabBlock in tests/scene_tab_reload.ps1: the steps from the issue. It packs over a scene open in another tab, checks the edited scene did not move, opens and saves the target, and reads the file. A second session closes the target and overwrites it with scene_create. All three lines pass locally. Before the switch back, 4.5.1 failed on the edited scene check.

Filed from this

• csharp_check_build calls a slow dotnet missing, because its version probe stops at 30 seconds #1078: csharp_check_build reports a slow dotnet as missing. It turned main red on the last run.
• scene_create over a scene open in another tab answers opened: false on Godot 4.5 and 4.6 #1079: scene_create over a scene open in another tab answers opened: false on 4.5 and 4.6.