Data classes exposed in public APIs might cause down-compatibility issues

Is your feature request related to a problem? Please describe.
The official Kotlin guidelines discourage use of data classes in public APIs:

https://kotlinlang.org/docs/api-guidelines-backward-compatibility.html#avoid-using-data-classes-in-your-api

This project is using data classes to express all the types of MCP protocol. This might cause all the down-compatibility issues described in linked document, if the MCP protocol evolves.

Describe the solution you'd like
A possible way to overcome this issues is to stop using data classes in favor of classes with private constructors, and allow to create them only with builders. If the MCP protocol is extended, the new class properties can be put in any order in the private constructor, to follow the protocol evolution. Example of such an approach here:

https://github.com/xemantic/xemantic-ai-tool-schema/blob/main/src/commonMain/kotlin/JsonSchema.kt

(e.g. ObjectSchema class). The tests are here:

https://github.com/xemantic/xemantic-ai-tool-schema/blob/main/src/commonTest/kotlin/JsonSchemaTest.kt

If this is considered a way to go, I would be happy to prepare this PR.

Start with the Kotlin backward-compatibility guidance linked in the issue, then review the SDK's MCP protocol types to identify which data classes are exposed publicly. Compare the proposed builder-based approach with the linked JsonSchema.kt and JsonSchemaTest.kt example; done means the affected public types preserve protocol behavior while avoiding the stated compatibility risks.