Add headless regression tests for Prop Editor functionality

- Implement tests for PropFile, including preset conversion, file dictionary conversion, round-trip serialization, version rejection, default values for missing keys, flattening, sanitization, and save/load functionality.
- Create tests for PropEditor, covering window title formatting, mode button text, safe saving, and recent file list management.
- Add tests for PropTheme, validating fallback behavior, invalid input handling, and default values for primitives and UI tokens.
- Introduce tests for ThumbnailCache, ensuring proper key formatting, PNG handling, and behavior with missing files.

tests/test_prop_editor_thumb.gd

@@ -0,0 +1,80 @@
+# Phase 9 Prop Editor - headless regression checks for the ThumbnailCache
+# extension (prop_file_key / prop_file_png) and PNG round-trip.
+# Run with:
+# & "C:\Godot4\Godot_v4.7.1-stable_win64_console.exe" --headless --path "C:\tmp\stickman" --script "res://tests/test_prop_editor_thumb.gd"
+
+extends SceneTree
+
+const THUMBNAIL_CACHE := preload("res://scripts/thumbnails/thumbnail_cache.gd")
+
+var _checks: int = 0
+var _failures: int = 0
+
+
+func _initialize() -> void:
+	call_deferred("_run")
+
+
+func _run() -> void:
+	print("\n=== PHASE 9 PROP THUMB TEST ===")
+	await _test_prop_file_keys()
+	await _test_builtin_prop_png_unchanged()
+	await _test_png_round_trip()
+	await _test_missing_file()
+	print("----------------------------------------")
+	if _failures == 0:
+		print("RESULT: ALL PASSED (%d assertions)" % _checks)
+		quit(0)
+	else:
+		print("RESULT: %d FAILURE(S) out of %d assertions" % [_failures, _checks])
+		quit(1)
+
+
+func check(label: String, condition: bool) -> void:
+	_checks += 1
+	if condition:
+		print("PASS: %s" % label)
+	else:
+		_failures += 1
+		print("FAIL: %s" % label)
+
+
+func _test_prop_file_keys() -> void:
+	var cache: ThumbnailCache = THUMBNAIL_CACHE.new()
+	var path := "user://prop_thumb_key_test.prop"
+	var f := FileAccess.open(path, FileAccess.WRITE)
+	f.store_string("{}")
+	f.close()
+	var key: String = cache.prop_file_key(path)
+	var mtime: int = FileAccess.get_modified_time(path)
+	check("prop_file_key format", key == "prop_thumb_key_test_%d" % mtime)
+	check("prop_file_png uses PROP_DIR", cache.prop_file_png(path) == "user://thumbnails/props/%s.png" % key)
+	check("prop_file_key missing file still formats", cache.prop_file_key("res://nope.prop") == "nope_0")
+	DirAccess.remove_absolute(path)
+
+
+func _test_builtin_prop_png_unchanged() -> void:
+	var cache: ThumbnailCache = THUMBNAIL_CACHE.new()
+	check("prop_png keeps version key", cache.prop_png("crate") == "user://thumbnails/props/crate_v%d.png" % THUMBNAIL_CACHE.PROP_VERSION)
+	check("stickman_png unchanged", cache.stickman_png("abc_123") == "user://thumbnails/stickmen/abc_123.png")
+
+
+func _test_png_round_trip() -> void:
+	var cache: ThumbnailCache = THUMBNAIL_CACHE.new()
+	var img := Image.create_empty(4, 4, false, Image.FORMAT_RGBA8)
+	img.fill(Color(1.0, 0.0, 0.0, 1.0))
+	var tex := ImageTexture.create_from_image(img)
+	var png_path := "user://thumbnails/props/test_roundtrip_v9.png"
+	var err: Error = cache.save_png(tex, png_path)
+	check("save_png succeeds", err == OK)
+	check("png exists on disk", FileAccess.file_exists(png_path))
+	cache.clear_memory()
+	var loaded: Texture2D = cache.load_png(png_path)
+	check("load_png round trip", loaded != null and loaded.get_size() == Vector2(4, 4))
+	check("save_png null texture rejected", cache.save_png(null, "user://thumbnails/props/null_v9.png") != OK)
+	DirAccess.remove_absolute(png_path)
+
+
+func _test_missing_file() -> void:
+	var cache: ThumbnailCache = THUMBNAIL_CACHE.new()
+	check("load_png missing -> null", cache.load_png("user://thumbnails/props/definitely_missing.png") == null)