Add SecretRedactionFilter to scrub API keys, tokens, and credentials
from all log records (messages, args, tracebacks, extra fields).
- Enable redaction by default; opt out with LITELLM_DISABLE_REDACT_SECRETS=true
- Redact patterns: sk-*, Bearer tokens, x-api-key values, base64 creds
- Handle JSON formatter exception hooks and percent-style format args
- Snapshot dict iteration to avoid RuntimeError during concurrent logging
