Fix source-created codegen variable reuse

[erdtsieck]

Summary

Fixes #391.

This updates MethodFrameArranger so a variable source that returns a concrete variable for an interface request is reused later when the concrete type is requested. The arranger now considers already-known created variables and their sibling cast variables before asking the same source to create another frame.

Root Cause

In the tenanted Wolverine.HTTP + Marten path, one frame can first request IMessageBus/IMessageContext, causing the source to create a concrete MessageContext frame plus cast variables. Later the outboxed Marten session frame requests the concrete MessageContext. The arranger only looked at top-level method frames before invoking sources again, so it built a second MessageContext frame and generated a duplicate var messageContext declaration.

Tests

• Added a regression test covering a source-created concrete context first requested through an interface and then requested by concrete type.
• dotnet test src\CodegenTests\CodegenTests.csproj --no-restore