test: add tests for virtual node filtering in API export

Add E2E and unit tests verifying that virtual nodes (Note,
MarkdownNote, Reroute, PrimitiveNode) are excluded from API format
export while preserved in standard workflow format.

Unit tests confirm graphToPrompt correctly filters isVirtualNode
from the output object. E2E tests verify the same through the
browser via getExportedWorkflow({ api: true }).

Includes a new test fixture (note_with_ksampler.json) combining
real and virtual nodes for mixed-workflow assertions.

browser_tests/assets/nodes/note_with_ksampler.json

@@ -0,0 +1,63 @@
+{
+  "last_node_id": 3,
+  "last_link_id": 0,
+  "nodes": [
+    {
+      "id": 1,
+      "type": "KSampler",
+      "pos": [400, 50],
+      "size": [315, 262],
+      "flags": {},
+      "order": 0,
+      "mode": 0,
+      "inputs": [
+        { "name": "model", "type": "MODEL", "link": null },
+        { "name": "positive", "type": "CONDITIONING", "link": null },
+        { "name": "negative", "type": "CONDITIONING", "link": null },
+        { "name": "latent_image", "type": "LATENT", "link": null }
+      ],
+      "outputs": [
+        { "name": "LATENT", "type": "LATENT", "links": [], "slot_index": 0 }
+      ],
+      "properties": { "Node name for S&R": "KSampler" },
+      "widgets_values": [42, "fixed", 20, 8, "euler", "normal", 1]
+    },
+    {
+      "id": 2,
+      "type": "Note",
+      "pos": [50, 50],
+      "size": [300, 150],
+      "flags": {},
+      "order": 1,
+      "mode": 0,
+      "inputs": [],
+      "outputs": [],
+      "properties": {},
+      "widgets_values": ["This is a reference note"],
+      "color": "#432",
+      "bgcolor": "#653"
+    },
+    {
+      "id": 3,
+      "type": "MarkdownNote",
+      "pos": [50, 250],
+      "size": [300, 150],
+      "flags": {},
+      "order": 2,
+      "mode": 0,
+      "inputs": [],
+      "outputs": [],
+      "properties": {},
+      "widgets_values": ["# Markdown heading"],
+      "color": "#432",
+      "bgcolor": "#653"
+    }
+  ],
+  "links": [],
+  "groups": [],
+  "config": {},
+  "extra": {
+    "ds": { "scale": 1, "offset": [0, 0] }
+  },
+  "version": 0.4
+}