feat: make structured mirroring explicit

[DaleSeo]

Motivation and Context

Automatic mirroring can double large structured payloads and makes a compatibility fallback an implicit SDK policy. This implements the alternative discussed in #1046: structured output is the default, and callers that need text content choose it explicitly without adding another return-type wrapper or macro behavior.

How Has This Been Tested?

Added integration tests

Breaking changes

Technically none because the API hasn't changed.

However, code that relies on structured, structured_error, or Json<T> to automatically populate content now needs to add the fallback explicitly with with_content or construct the CallToolResult directly.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

[alexhancock]

cc @olaservo can you review?