[Core] The `ChatClient` auto-configuration doesn't back off when a `ChatClient.Builder` bean exists already

[ThomasVitale]

Bug description
The ChatClient.Builder bean auto-configured in ChatClientAutoConfiguration is created even though a bean already exists (defined custom by an application).

Environment
Spring AI 1.0.0-SNAPSHOT
Java 23

Expected behavior
When a custom ChatClient.Builder bean is defined by an application, the default auto-configured one should not be created (the @ConditionalOnMissingBean should be added). That is in line with how other Builder beans are configured in Spring Boot (e.g. RestClient.Builder).