Document Spring AI raw provider diagnostics hardening

[liuyun71378]

Summary

• keep Spring AI external raw provider diagnostics defaulting to warn
• preserve fail and off as explicit modes, with fail documented as opt-in production hardening
• add coverage for the default warn behavior and explicit fail behavior
• update the Spring AI boot starter example docs to match the compatibility boundary

Tests

• JAVA_HOME=/Users/liuyun/Library/Java/JavaVirtualMachines/ms-21.0.10/Contents/Home mvn -pl fastmcp-spring-ai-boot-starter -am -Dtest=FastMcpSafeAutoConfigurationTest,SpringAiExternalRawProviderDiagnosticsTest -Dsurefire.failIfNoSpecifiedTests=false test
• JAVA_HOME=/Users/liuyun/Library/Java/JavaVirtualMachines/ms-21.0.10/Contents/Home mvn test
• JAVA_HOME=/Users/liuyun/Library/Java/JavaVirtualMachines/ms-21.0.10/Contents/Home mvn -Pexamples test