Add regression test for JsonSerializer.Deserialize<JsonNode> respecting MaxDepth with deeply nested arrays

[Copilot]

JsonSerializer.Deserialize<JsonNode> correctly enforces MaxDepth via Utf8JsonReader's depth tracking in StartArray()/StartObject(), but there was no test covering this deserialization path — only the serialization direction (SerializeToNode) was tested, leaving the door open for regressions.

Adds DeserializeToNode_RespectsMaxDepth to DomTests.cs, mirroring the existing SerializeToNode_RespectsMaxDepth test:

[Theory]
[InlineData(5)]
[InlineData(32)]
[InlineData(70)] // default max depth is 64
public static void DeserializeToNode_RespectsMaxDepth(int maxDepth)
{
    var options = new JsonSerializerOptions { MaxDepth = maxDepth };

    // Exactly at the limit succeeds
    string validJson = string.Concat(Enumerable.Repeat("[", maxDepth))
                     + string.Concat(Enumerable.Repeat("]", maxDepth));
    Assert.NotNull(JsonSerializer.Deserialize<JsonNode>(validJson, options));

    // One level over throws
    string tooDeepJson = string.Concat(Enumerable.Repeat("[", maxDepth + 1))
                       + string.Concat(Enumerable.Repeat("]", maxDepth + 1));
    Assert.Throws<JsonException>(() => JsonSerializer.Deserialize<JsonNode>(tooDeepJson, options));
}