* feat(tier): add transition candidate probe contract
Add a fail-closed WarmBackend probe contract for provider-authoritative transition candidate state. Default providers report Unsupported, while the shared mock backend can now model missing, unversioned, and exact-version candidates for follow-up recovery tests.
This is a forward-compatible foundation for #1352/#1358 recovery work and does not change production cleanup behavior.
Co-Authored-By: heihutu <heihutu@gmail.com>
* feat(tier): probe transition candidates from providers
Implement provider-authoritative transition candidate probing for S3-family warm backends by querying ListObjectVersions with exact-key filtering and fail-closed classification for delete markers, multiple versions, truncation, and unknown versioning state.
This keeps non-S3 providers on the default Unsupported probe result and forwards MinIO, RustFS, and R2 through the S3 probe implementation.
Co-Authored-By: heihutu <heihutu@gmail.com>
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Co-authored-by: heihutu <heihutu@gmail.com>
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