[AIP-94] Route airflow backfill create through the API server

[yuseok89]

relates: #68402

Routes airflow backfill create through the API server via the airflowctl HTTP client (no more direct metadata-DB access) and deprecates it in favour of the existing airflowctl backfill create. --dry-run now hits the backfills/dry_run
endpoint; a latent wrong return type in the airflowctl BackfillOperations.create_dry_run is fixed so that path works.

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)
  • Opus 4.8

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