Backport of fix: consume ignored entries in CE downgrade via Ent snapshot into release/1.18.x

[dhiaayachi]

Backport

This PR is auto-generated from hashicorp#20977 to be assessed for backporting due to the inclusion of the label backport/1.18.

The below text is copied from the body of the original PR.

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Description

This operation would previously fail due to unconsumed bytes in the decoder buffer when reading the Ent snapshot (the first byte of the record would be misinterpreted as a type indicator, and the remaining bytes would fail to be deserialized or read as invalid data).

Ensure restore succeeds by decoding the ignored record as an interface{}, which will consume the record bytes without requiring a concrete target struct, then moving on to the next record.

Opened in response to a bug report by @sayap (thank you)

Testing & Reproduction steps

• Added new test to verify Ent->CE restore works as intended

PR Checklist

• updated test coverage
• external facing docs updated
• appropriate backport labels added
• not a security concern

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Overview of commits

• 24f226e