fix(etcdctl): parse lease txn compare values

[loingtan]

Summary

• Fix etcdctl txn parsing for lease comparisons such as lease("foo") > "0".
• Parse lease compare values as integers before constructing the client compare, matching revision/version compare parsing.
• Add regression coverage for lease compare parsing.

Fixes #20773

Test Plan

• go test ./etcdctl/... -count=1

Distributed systems note

Lease comparisons are part of etcd transaction conditions; this keeps CLI transaction parsing aligned with the lease ID semantics used by the client compare API.

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[k8s-ci-robot]

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This pull-request has been approved by: loingtan
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