[Feature]: Support for outputSchema in Tool interface

### Pre-flight Checklist

- [x] I have searched existing issues and this hasn't been requested yet.

### Problem or Motivation

Hello,

It is possible in [MCP tools](https://modelcontextprotocol.io/specification/2025-06-18/server/tools#tool) to provide both an input schema and an [output schema](https://modelcontextprotocol.io/specification/2025-06-18/server/tools#output-schema).

But the [`Tool` interface](https://github.com/ag-ui-protocol/ag-ui/blob/b80612d9bd7a25a13df3aaa751ff048dfe5b8c7a/sdks/typescript/packages/core/src/types.ts#L186-L191) exposed by AG-UI:
- does not use the same property names as the MCP spec
- in particular, does not support `outputSchema`

Would it be possible to add support for `outputSchema` ? I believe this could be useful to let the LLM know what it can expect as output of a given tool call.

Also, why not align on the MCP spec ?

Thank you !

### Proposed Solution

Align on the MCP spec for tools, and in particular support `outputSchema`.

### Alternatives Considered

_No response_

### Additional Context

_No response_