* fix(guardrails): return 400 not 500 when AIM blocks a request
AIM guardrail blocks raised a bare HTTPException whose type and param
serialized as the literal string "None", which broke OpenAI-SDK error
parsing for downstream consumers. Switching AIM to raise a ProxyException
surfaced a second bug: the shared error funnel re-derived the HTTP status
from a nonexistent status_code attribute and downgraded the 400 to a 500.
The funnel now honors an already-normalized ProxyException rather than
rebuilding it, and ProxyException is excluded from llm_exceptions alerting
so a content-policy block no longer pages on-call as an LLM API failure
Resolves LIT-3751
* fix(guardrails): route all AIM rejection paths through ProxyException
The block-action fix left two AIM rejection paths raising a bare
HTTPException: the multimodal anonymize rejection and the output-side
block. Both serialized type and param as the literal string "None", the
same malformed shape the block fix removed. Funnel all three through a
shared _rejection helper so they return a conformant OpenAI error body.
The output block carries content_policy_violation; the multimodal
rejection stays a plain invalid_request_error because it is a usage
error, not a policy violation
Resolves LIT-3751
* fix(guardrails): record AIM ProxyException blocks in failure logs
Switching AIM blocks from HTTPException to ProxyException made
_is_proxy_only_llm_api_error return False for them, so
_handle_logging_proxy_only_error was skipped and the blocked prompt was
dropped from the configured failure loggers. Classify ProxyException as a
proxy-only error alongside HTTPException so guardrail blocks are recorded
again, matching the prior behavior. The llm_exceptions alert suppression
is a separate check and stays in place
Resolves LIT-3751
* style(guardrails): use str | None over Optional[str] in AIM _rejection
* style(guardrails): collapse AIM _rejection signature per black
(cherry picked from commit b5fcd859be)