fix(spend-logs): redact echoed prompts in error_information (LIT-2992) (BerriAI#27689)

Provider validation errors (e.g. OpenAI RateLimitError carrying 178
pydantic errors each with their own 'input': [...]) were stored verbatim
in LiteLLM_SpendLogs.metadata.error_information.error_message via
str(original_exception), producing rows >12 MB.

Sanitize before metadata is serialized:
- redact 'input'/'messages' values in both error_message and traceback
  when store_prompts_in_spend_logs is False (back-door leak paths)
- always apply the MAX_STRING_LENGTH_PROMPT_IN_DB size cap to
  error_message and traceback (DB-storage safeguard)

Value scanning uses a parser-based balanced-bracket walk that respects
string quoting, so multi-modal payloads ('messages': [{'content': [...]}])
and user text containing literal brackets ("secret[123") are handled
correctly instead of leaking past a depth-1 regex.

Scoped to the spend-log path so OTEL/Datadog/etc. callbacks still
receive the untruncated error per LITELLM_TRUNCATION_DB_SAFEGUARD_NOTE.

Co-authored-by: Yassin Kortam <yassinkortam@g.ucla.edu>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: 3 people