fix: prevent stack overflow on deeply nested JSON payloads

[nfvelten]

Summary

• scan_value in payload_filter.rs was fully recursive and would overflow the stack on pathologically nested JSON arguments. Replaced with an explicit-stack iterative traversal capped at MAX_DEPTH = 64.
• redact_value in gateway.rs was also recursive; refactored to depth-parameterised recursion (redact_value_depth) that returns the value unchanged beyond REDACT_MAX_DEPTH = 64 instead of panicking.
• Added two unit tests: one verifying deeply nested input (MAX_DEPTH+10) is blocked, one verifying input exactly at MAX_DEPTH is allowed through.

Closes #95

Test plan

• cargo test --lib — all 431 unit tests pass
• cargo clippy -- -D warnings — zero warnings
• cargo fmt --check — no formatting violations
• New test deeply_nested_json_blocked — nesting beyond MAX_DEPTH triggers block
• New test max_depth_exactly_allowed — nesting at exactly MAX_DEPTH passes through

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