Downgrade checkpoint fallback log from error to warn

[aluzzardi]

Downgrade checkpoint fallback log from error to warn

---

Note

Downgrades the log level for checkpoint fallback from Error to Warn in AppendOp, reflecting that falling back to async response is a handled/expected degradation path rather than an unrecoverable error.

^{Written by Mendral for commit a4602ee.}

[changeset-bot]

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Latest commit: a4602ee

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[mendral-app]

LGTM

Change is correct — falling back to async response is a recoverable path, so Warn is the appropriate severity. No security, correctness, or data loss concerns.

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