client/v3: check lease freshness before serving cached reads, fix EvictRange bug

[thecitymouse]

The leasing layer claims linearizable reads via lease-based escape, but never actually checks the lease is valid before serving from cache. Adds the missing freshness check. Cannot be fully safe under unbounded clock drift, which matches what the package docs already imply about lease-based mechanisms.

Also fixes EvictRange using the loop parameter key instead of the loop variable k.

Test case from @upamanyus in #19091 / #19092.

Fixes #19091

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