feat: retry a workflow-as-code task from its task options (#11013)

* feat: retry a workflow-as-code task from its task options

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WvjgKMRNtRNPnAkg6MkiTA

* fix: claim every retry attempt key up front, so a step cannot alias one

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WvjgKMRNtRNPnAkg6MkiTA

* fix: bound retry attempts, which now claim their keys up front

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WvjgKMRNtRNPnAkg6MkiTA

* fix: honour an explicit zero retry multiplier in the python client

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WvjgKMRNtRNPnAkg6MkiTA

* docs: state the retry validation rules once in the task docstring

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WvjgKMRNtRNPnAkg6MkiTA

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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>