fix(ai): restore WebClient runtime auto-configuration

[JasonTM17]

Summary

• replace the legacy WebFlux starter with Spring Boot 4's dedicated WebClient starter
• restore the auto-configured WebClient.Builder required by the Anthropic and job-search clients
• add a regression test for both AI client beans

Evidence

• reproduced the master failure: AI service exited because WebClient.Builder was missing
• mvn -B -pl ai-service -am verify
• rebuilt ai-service in Docker and verified /actuator/health/readiness returns UP