Fix ToolCallback overload selection in ChatClient

[juanjuandog]

A direct ToolCallback argument to tools(...) binds to Object... today. That sends FunctionToolCallback through the @tool method path and ends with "No @tool annotated methods found".

This adds the specific ToolCallback and ToolCallbackProvider overloads for tools(...) and defaultTools(...), routed through the existing ToolSpec callback path.

Closes #2495

Tests:

• ./mvnw -Dmaven.build.cache.enabled=false -pl spring-ai-client-chat -Dtest=DefaultChatClientTests#whenToolCallbackPassedDirectlyToToolsThenReturn,DefaultChatClientBuilderTests#whenToolCallbackPassedDirectlyToDefaultToolsThenReturn test
• ./mvnw -Dmaven.build.cache.enabled=false -pl spring-ai-client-chat test

[juanjuandog]

This still looks clean from my side. The branch is up to date and the checks are green, so I think this is ready for review when you have a chance.