Failure analysis for a missing mail sender is misleading

[answndud]

Summary

• add a mail-specific failure analyzer for missing MailSender beans when mail auto-configuration does not match
• provide guidance for configuring spring.mail.host or spring.mail.jndi-name instead of only suggesting a custom bean
• add focused tests for the analyzer and register it in spring.factories

Testing

• ./gradlew :module:spring-boot-mail:test --tests org.springframework.boot.mail.autoconfigure.NoSuchMailSenderBeanFailureAnalyzerTests

Fixes #49359

[wilkinsona]

Thank you, @answndud.