agentHost: re-enable retryable errors (#333260)

* Revert "agentHost: temporarily disable retryable errors"

Reverts #332839 now that the retry issue is fixed.

(Written by Copilot)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* agentHost: preserve non-resumable error handling

Keep subagent errors non-resumable when their parent mapping is missing, and retain dispatcher coverage for rejecting non-resumable retry requests.

(Written by Copilot)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>