Conformance plan for Interceptor

We currently have multiple Interceptor SDKs implementing the spec but disagreeing slightly on protocol details. These SDKs all have examples, test fixtures, and tests which exercise the protocol via the SDK, which prove that the SDK works, but not that it implements the protocol or would interoperate with a solution using a different SDK.

We should have a conformance strategy for Interceptors that is consistent with https://github.com/modelcontextprotocol/conformance - at least in part because if Interceptors moves into the core spec at some point, the conformance testing will move there.

Once we have a solid direction, the SDKs will need to be updated to support conformance and guidance will need to be produced for any new SDKs so they implement conformance support properly.

Lastly, while the core conformance effort explicitly chose not to matrix test clients and servers across SDKs (in favor of deterministic results with clear responsibility), I think it might be good with these early Interceptor SDKs (while there are still only a few of them) to matrix tests them just to prove to ourselves that they interoperate and our conformance testing isn't missing anything.

Note: This issue came up in Interceptors WG meeting and I'm documenting it here just so we don't lose track of it.

Start by reviewing the linked modelcontextprotocol/conformance project and the existing Interceptor SDK examples, test fixtures, and tests. Define a conformance strategy and guidance for SDK support, then identify how interoperability can be validated across the currently available SDKs; done means the direction is agreed and actionable.