fix(ui): add traversal depth and result count bounds for UI hierarchy and queries (#126)

[Daliys]

Summary

Addresses #126 by adding bounds and truncation indicators to UI Toolkit VisualElement hierarchy traversal and queries.

Problem

UI Toolkit traversal for hierarchy and element query tools (ui_get_hierarchy, ui_query_elements, ui_capture_window_snapshot) recursively walked full VisualElement trees synchronously on the Unity main thread without depth or result count limits, causing visible editor stalls and oversized JSON payloads on complex windows.

Changes

• SerializeVisualElement: added maxDepth (default: 30) and maxElements (default: 1000) caps. If limits are reached, recursion is halted, the truncated element node is marked with children_truncated: true, and the root object is marked with truncated: true.
• UIGetHierarchy: accepts optional max_depth and max_elements / max_results parameters.
• UIQueryElements & QueryVisualElementRecursive: accepts optional max_depth (default: 30) and max_results (default: 100). Immediately aborts recursion when maxResults or maxDepth is hit.
• UICaptureWindowSnapshot: forwards max_depth and max_elements to hierarchy serialization.
• MCP Bridge (routing.py, schemas.py): exposed and forwarded max_depth, max_elements, and max_results on unity_ui_automation (get_hierarchy, query, capture_window_snapshot).
• Documentation & Specs: updated API_REFERENCE.MD, DOCUMENTATION.MD, and CHANGELOG.md.
• Tests: added C# tests in AgentToolingTests.cs and Python tests in test_routing.py and test_schemas.py.

Closes #126