fix!: persist structured chat errors by ethanndickson · Pull Request #24919 · coder/coder

Drop the legacy `last_error` headline string and rename `last_error_payload` to `last_error` so chats expose a single structured error object. Make persisted decode lossless, share one frontend normalizer between live and persisted paths, guard the HTTP chip on positive status codes, and align tests, stories, fixtures, and docs.

Removes leftover compatibility scaffolding from the chat last-error
redesign:

- normalizeChatErrorPayload no longer takes a fallbackMessage option;
  the live-stream "Chat processing failed." default lives at the only
  caller that needs it, and the redundant statusCode > 0 check in
  ChatStatusCallout is gone now that the normalizer filters bad codes.
- Deletes chaterror.StreamErrorPayload, which was a thin alias for
  TerminalErrorPayload, and inlines the chatLastErrorMessage helper in
  cli/agents_list.go to match the file's existing nil-check style.
- Renames legacyPayload to wantPayload in chatd_test.go and corrects
  stale "migration 474" comments in the chat last-error fixtures.
- Adds a structured last_error example to docs/ai-coder/agents/chats-api.md.

decodeChatLastError, the cached-error precedence in AgentChatPage, the
encodeChatLastErrorPayload helper, and the LastError clears on chat
status transitions are intentionally unchanged.