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Fix flag completion ( 1438)

Fix flag completion ( 1438): a task in repo2rlenv-commit-runtime-v2 (Harbor dataset). Title: Flag completion broken when RegisterFlagCompletionFunc is called before command is added to parent

The task

**Title:** Flag completion broken when `RegisterFlagCompletionFunc` is called before command is added to parent

Part of FineEnvs/repo2rlenv-commit-runtime.