Attribute guardrail blocks in telemetry for /v1/responses and /v1/embeddings (Blocked tab under-counts)

Summary

/v1/chat/completions threads applied_guardrails and sets guardrail_blocked=true on the UsageEvent so the dashboard's Blocked tab reflects guardrail blocks. /v1/responses and /v1/embeddings (input guardrails added in #541) emit only a generic zero-token error event (error_class=content_filter) on a block — no guardrail_blocked flag and no applied_guardrails set.

Impact

Low (observability only): the dashboard Blocked tab and applied-guardrail analytics under-count guardrail blocks on the non-chat surfaces. The embeddings handler already documents that bypass-telemetry plumbing for non-chat handlers is deferred.

Fix

Thread applied_guardrails + a guardrail_blocked signal through the responses/embeddings dispatch return + emit paths, mirroring chat.rs.

Surfaced by the independent audit of #541. Refs #719.

Start with chat.rs to trace how applied_guardrails and guardrail_blocked reach UsageEvent, then follow the /v1/responses and /v1/embeddings dispatch return and emit paths. Confirm how input-guardrail blocks currently produce the zero-token content_filter event. Done means both non-chat surfaces record the applied guardrails and guardrail_blocked signal so the dashboard Blocked tab and guardrail analytics include those blocks.