JSONLContent parses each line as JSON to determine which string values
need redaction, then performs targeted replacements on the raw JSON bytes.
Lines with no secrets are returned unchanged, preserving original formatting.
String replaces secrets in s with "REDACTED" using layered detection:
1. Entropy-based: high-entropy alphanumeric sequences (threshold 4.5)
2. Pattern-based: gitleaks regex rules (180+ known secret formats)
A string is redacted if EITHER method flags it.