fix(pd): handle deep prompt-cache keys iteratively

[WANDY666]

Summary

• replace recursive prompt-cache trie insertion and matching with iterative traversal
• remove the process-wide sys.setrecursionlimit configuration
• backport the existing support_ds4 fix from commit 7bc0955b20

Deep sampled prompt-cache keys can still exceed any fixed Python recursion limit. Iterative traversal removes that limit without changing the trie/LRU behavior.

Validation

• python -m pytest -q -p no:cacheprovider unit_tests/server/test_pd_cache_aware.py — 12 passed
• inserted and matched a 10,000-character key with Python's recursion limit left at 1,000
• black --check --line-length=120 on both changed files
• verified the backport patch-id matches the original commit