Stacked on #27477. That PR puts the actionable provider detail into the wait_agent last_error string; this PR additionally exposes the decoded ChatError fields as structured siblings so the parent model does not have to parse prose to decide whether respawning makes sense.

Changes

• Terminal wait_agent error payloads gain, when the persisted last_error decodes as a ChatError:
  • last_error_kind (defaults to generic when empty)
  • last_error_retryable (explicit bool)
  • last_error_detail (only when non-empty)
• last_error stays a string with the semantics from fix(coderd/x/chatd): surface child error detail in wait_agent last_error #27477. Malformed payloads keep the raw-JSON string and get none of the structured fields.
• subagentLastErrorMessage becomes subagentLastError, returning the decoded payload alongside the display string so decoding happens once for both the poll and timeout-recheck paths.
• Exact assertions for populated, message-only, and malformed cases on both paths.

No other tool payloads change, and no repository consumer parses the wait_agent response shape, so the fields are additive.

Validation

• go test ./coderd/x/chatd/..., make lint/go green.
• Dogfood UAT on a dev instance: parent wait_agent results carried last_error_kind/last_error_retryable/last_error_detail for a detail fixture, and correctly omitted last_error_detail for a message-only fixture.

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