fix(parse): build array String() without recursion

utils.generation_params converted nested arrays to strings recursively,
so a crafted file with a value nested about 900 arrays deep raised
RecursionError, where the browser parser returns a result. The join now
runs on an explicit stack. A JS-parity case with 900-deep values pins it.

tests/generation_params_cases.py

@@ -244,6 +244,14 @@ def _workflow_only():
     }
 
 
+def _nested(depth):
+    """A value nested ``depth`` arrays deep, as a crafted file can hold."""
+    value = ["leaf", 7]
+    for _ in range(depth):
+        value = [value, None]
+    return value
+
+
 def _chain(length):
     graph = {"4": CHECKPOINT, "1": encoder("deep chain text")}
     previous = "1"
@@ -440,5 +448,14 @@ CASES = {
             "3": ksampler(("6", 0), ("6", 0), cfg=1.0, seed=4974052037430000001),
         }
     ),
+    "deeply-nested-values": case(
+        {
+            "4": CHECKPOINT,
+            "6": encoder("nested neighbours"),
+            "3": ksampler(("6", 0), ("6", 0)),
+            "8": {"class_type": _nested(900), "inputs": {"text": "x"}},
+        },
+        {"nodes": [{"id": _nested(900), "type": _nested(900)}], "links": []},
+    ),
     "nothing": case(),
 }

tests/test_generation_params.py

@@ -176,6 +176,12 @@ class StaysBounded(unittest.TestCase):
         self.assertEqual(result["negativePrompt"], "t")
 
 
+class DeeplyNestedValues(unittest.TestCase):
+    def test_deeply_nested_values_do_not_exhaust_the_python_stack(self):
+        result = run("deeply-nested-values")
+        self.assertEqual(result["positivePrompt"], "nested neighbours")
+
+
 class Placeholders(unittest.TestCase):
     def test_blank_text_is_a_placeholder_and_zero_seed_is_kept(self):
         result = run("blank-text-and-zero-seed")