fix: align Spring smoke test protobuf runtime

[zeitlinger]

Summary

• pin the Spring Boot smoke test's Protobuf runtime to the version used to generate the client sources
• manage the dependency explicitly so Renovate can keep the test version aligned on future Protobuf updates

Root cause

The Spring Boot parent currently manages protobuf-java at 4.34.2, while the client sources are generated with 4.35.1. During a reactor test, Maven uses the exposition module's target/classes before shading, so the generated 4.35.1 classes load against Spring Boot's older 4.34.2 runtime and fail the runtime compatibility check.

Published artifacts remain shaded; this change only aligns the reactor smoke-test classpath.

Validation

• reproduced on main with ./mvnw clean test -pl integration-tests/it-spring-boot-smoke-test -am -Dcoverage.skip=true -Dwarnings=-nowarn
• mise run lint:fix
• mise run test