* fix(proxy): invoke post-call guardrails on pass-through endpoint responses (#20270)
Wire post_call_success_hook into non-streaming pass-through response path,
gated on explicit guardrail config (opt-in only, no backwards-compat break).
- Call post_call_success_hook after reading non-streaming response body
- Build enriched hook_data with guardrails metadata and litellm_logging_obj
at call site (avoids mutation of _parsed_body which is shared by logging)
- Handle ModifyResponseException with provider-agnostic error envelope,
post_call_failure_hook, and defensive try/except
- Strip stale content-length when guardrail modifies response body
- Move ModifyResponseException to litellm.exceptions to break cyclic import;
re-export from custom_guardrail for backwards compat
- Add call_type fallback in UnifiedLLMGuardrails for pass-through endpoints
using CallTypes.pass_through.value enum
* test: add unit tests for pass-through post-call guardrails
5 tests covering the post-call guardrail invocation on pass-through endpoints:
- post_call_success_hook fires when guardrails configured
- post_call_success_hook skipped when no guardrails (backwards compat)
- ModifyResponseException returns 200 with provider-agnostic error
- UnifiedLLMGuardrails resolves call_type from logging_obj for pass-through
- ModifyResponseException re-export from custom_guardrail stays in sync
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