Update MCP implementation to support structured tool output

[tenntenn]

Background

The latest MCP specification (2025-06-18) added support for structured tool output, but the current go-sdk implementation still uses workarounds.

Current Implementation

• Using TextContent with manually marshaled JSON
• CallToolResultFor[T] doesn't properly handle structured responses
• go-sdk has TODO comment: "check that structured content is set in response"

Goal

When go-sdk catches up with the latest MCP specification:

• Replace manual JSON marshaling with native structured responses
• Remove mustMarshalJSON workaround
• Use proper structured content types

References

• MCP Specification Changelog: https://modelcontextprotocol.io/specification/2025-06-18/changelog
• Structured tool output was added in PR #371