fix(rmcp): return serialized json with structured content

[ccamel55]

Motivation and Context

Issue #355 seems to be about using this crate as a client, however the contrary also needs to be addressed.

The 2025-06-18 specification states the following recommendation:

For backwards compatibility, a tool that returns structured content SHOULD also return the serialized JSON in a TextContent block.

This issue can be observed when trying to use tasks which return structured_content with clients that don't support 2025-06-18 yet.

How Has This Been Tested?

Unit tests check:

• both content and structured_content exist for structured results
• content contains serialized version of structured_content

Breaking Changes

No breaking changes.

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

Additional context

• https://modelcontextprotocol.io/specification/2025-06-18/server/tools#structured-content
• fix(model): allow backwards compat as per MCP spec #357 PR for fixing issue previously mentioned

[4t145]

Just have a look on https://modelcontextprotocol.io/specification/2025-06-18/server/tools

This should be the correct implementation.