[ISSUE]: Depth validation uses recursion, enabling CPU/resource exhaustion #471

Description

Issue: _validate_depth() recursively traverses the entire JSON structure. Even with a depth cap, large payloads can still burn significant CPU/time and recursion overhead.

Fix: Replace recursion with an iterative traversal (explicit stack/queue), short-circuit immediately when depth exceeds MAX_NESTED_DEPTH, and add a performance-oriented test.

Additional Context

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