Comparing value in TXN on non-existing key always returns failure.

[AkashKumar7902]

Refs: #19678

What changed

• Clarify that a value comparison against a missing key, or a range containing no keys, evaluates to false regardless of the comparison operator.
• Regenerate the Go protobuf and Swagger artifacts from rpc.proto.

This updates the canonical API documentation source. Publishing the generated API reference to the website remains follow-up work, so this PR references rather than closes the issue.

Validation

• ./scripts/genproto.sh
• make verify-proto-annotations
• go test ./api/etcdserverpb
• go test ./server/etcdserver/txn
• jq empty Documentation/dev-guide/apispec/swagger/rpc.swagger.json
• gofmt -d api/etcdserverpb/rpc.pb.go
• git diff --check upstream/main...HEAD

[kubernetes-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[kubernetes-prow]

Hi @AkashKumar7902. Thanks for your PR.

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