GH-6206: Update tool callback guidance

[YunKuiLu]

Summary

I'm not sure whether the maintainers want to add tools(ToolCallback...) and tools(ToolCallbackProvider...) overloads. This PR does not add those APIs. Instead, it updates the exception message and documentation examples that currently point users toward deprecated or incorrect usage.

Changes

• Update the MethodToolCallbackProvider exception message to suggest using tools(toolSpec -> toolSpec.callbacks(...)) instead of passing callbacks/providers directly to tools(...).
• Update MCP and tool migration documentation examples to pass ToolCallback and ToolCallbackProvider instances through callbacks(...).
• Fix a ToolCallingChatOptions example that passed an @Tool object directly to toolCallbacks(...) by converting it with ToolCallbacks.from(...).

Tests

• ./mvnw -pl spring-ai-model -Dtest=MethodToolCallbackProviderTests test

Related #6206