AIP-78: Fix airflowctl backfill management API methods

[Sangun-Lee-6]

Fix airflowctl backfill management operations to use the HTTP methods defined by the public API.

The public API exposes backfill pause, unpause, and cancel as PUT endpoints. This updates airflowctl to match the API contract and extends unit tests to verify request methods in addition to paths.

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

• Yes — Codex (GPT-5)

Generated-by: Codex (GPT-5) following the guidelines

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