Add OutputSchema property to McpServerToolCreateOptions and McpServerToolAttribute

Adds support for specifying the output schema type independently of the return
type for tools returning CallToolResult. This allows developers to advertise
a meaningful OutputSchema in tools/list while still having full control over
the CallToolResult response (Meta, IsError, StructuredContent).

- Add `JsonElement? OutputSchema` property to McpServerToolCreateOptions
- Add `Type? OutputSchema` property to McpServerToolAttribute
- DeriveOptions converts the attribute's Type to a JSON schema
- CreateOutputSchema uses explicit OutputSchema over AIFunction's return schema
- Add 13 comprehensive tests covering all variations

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot