fix(coderd/x/chatd): surface child error detail in wait_agent last_error - #27477

When a child subagent chat fails, its persisted last_error carries both a normalized message and an actionable provider detail. The wait_agent tool result returned only the message, which for unclassified failures is the generic fallback "The chat request failed unexpectedly." The detail (for example reasoning model `max` not supported) never reached the parent model, so it retried blind instead of self-correcting (observed in dogfood chat 37c516d2: the parent respawned a kimi-k3 child with max effort twice and needed human intervention to switch to xhigh).

Changes

• subagentLastErrorMessage now builds the last_error string with the detail: detail alone when the message is empty or the generic fallback, message (detail) when both are meaningful, and message alone when there is no detail. This mirrors exactly what the chat UI already renders from the same persisted payload; no content is added or removed relative to what the user sees in the chat window.
• Unrecognized payloads (raw bytes that do not decode as a ChatError) no longer surface at all; the handler falls back to its status reason instead of exposing raw stored JSON.
• Unit table test for the helper plus regression tests asserting the detail-aware last_error on both the normal wait_agent poll path and the timeout TOCTOU recheck path.

Both paths share the same response block, so no other tool payloads change.

An earlier revision added read-path credential redaction (auth-detail suppression plus scrubbing regexes) on top of this. It was removed: the same detail is already served verbatim to the same user through the chats API and rendered in the chat window, so a wait_agent-only scrub added inconsistency without a security boundary, and its opaque-token heuristic destroyed useful diagnostics such as request IDs. If provider error bodies echoing credentials ever becomes a demonstrated problem, the fix belongs at classification/write time in chaterror so every surface stays consistent.

Validation

• go test ./coderd/x/chatd/..., make lint/go green.
• Dogfood UAT on a dev instance: injected the production fixture into a child chat and confirmed the parent's wait_agent result now carries reasoning model `max` not supported instead of the generic message.

This PR was authored by Mux, an AI coding agent, operating on Mike's behalf.