Backfills trace injection test and migrates from request.params to request.params._meta

[codefromthecrypt]

Motivation and Context

Before, trace IDs were encoded in the request.params object which co-mingled it with fields like tool arguments.

This backfills an encoding test and moves it to request.params._meta which is used elsewhere and proposed as a convention due to prior art and also being a bit safer for ad-hoc implementation regardless of this implementation being careful.

This also corrects logic to replace any existing key in the message however unlikely that might be.

How Has This Been Tested?

Backfilled a unit test

Breaking Changes

The choice to use request.params was released in https://github.com/modelcontextprotocol/csharp-sdk/releases/tag/v0.1.0-preview.9. This changes it to request.params._meta. This means that applications already releasing this will need to update both sides to have context propagation. Since this SDK is in pre-release, I didn't add logic to fall back to extract from request.params

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

• change that introduced use of request.params OpenTelemetry: context propagation and semconv update #262
• proposal to document request.params._meta for use in opentelemetry traces as well other metadata: Document request.params._meta convention modelcontextprotocol#414

[stephentoub]

cc: @lmolkova, @samsp-msft

[lmolkova]

Thank you!

[samsp-msft]

woot!