Cancel background task runners on server disposal

[jstar0]

Summary

Background Tasks runners used cancellation sources independent of the owning
server and discarded their Task handles. As a result, server disposal could
finish before a runner was cancelled, including when the runner registered
during disposal.

Changes

• Add a server-lifetime cancellation and background-task registration seam for
  extension infrastructure.
• Cancel the lifetime at the start of stateful server disposal, close
  registration after request handlers drain, and await the remaining runners.
• Link Tasks and long-lived MRTR handlers to the server lifetime while removing
  completed registrations.
• Keep stateless HTTP request servers non-owning so Tasks can continue across
  independent requests.
• Add deterministic coverage for cancellation, awaiting, late registration,
  and the stateless ownership boundary.

The change does not alter protocol messages, task-store contracts, polling, or
explicit tasks/cancel behavior.

Verification

dotnet test tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj --framework net10.0
dotnet test tests/ModelContextProtocol.AspNetCore.Tests/ModelContextProtocol.AspNetCore.Tests.csproj --framework net10.0
dotnet test tests/ModelContextProtocol.Analyzers.Tests/ModelContextProtocol.Analyzers.Tests.csproj --framework net9.0
dotnet build ModelContextProtocol.slnx --configuration Release
dotnet pack ModelContextProtocol.slnx --configuration Release --no-build
dotnet docfx docs/docfx.json --warningsAsErrors true

Results: Core 2290 passed; ASP.NET Core 507 passed; analyzers 57 passed;
package validation and DocFX succeeded.

Closes #1707