Add builder pattern to AssistantMessage and ToolResponseMessage

[JM-Lab]

Unlike UserMessage and SystemMessage, AssistantMessage and ToolResponseMessage did not support the builder pattern. This PR applies the builder pattern to these classes for consistent and flexible object creation.
Additionally, the properties field in AssistantMessage has been renamed to metadata for consistency with other message types.

• Refactor affected usages after AssistantMessage and ToolResponseMessage update
• Added builder pattern to AssistantMessage and ToolResponseMessage.
• Renamed the properties field to metadata in AssistantMessage for improved semantic clarity and consistency.

[ericbottard]

Hi, thanks for taking the time to contribute to spring-ai!

Please see #4457

AssistantMessage already supports builders on main.
Could you please re-work this PR to address ToolResponseMessage only (or feel free to close and open a new PR).

The agreed upon approach we use / aim for for the Builder pattern is:

• builder is final with private properties
• if there is a class hierarchy, the BBuilder doesn't extend from ABuilder