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/
dotnet test tests/ModelContextProtocol.Analyzers.Tests/
dotnet build
dotnet pack
dotnet docfx docs/docfx.json --warningsAsErrors true

The lifecycle focus passed on net10.0, net9.0, and net8.0 (3 tests on each
framework), with 20 additional consecutive net10.0 repetitions. The full Core
suite passed with 2312 passed and 5 skipped; the analyzer suite passed with 57
passed. The Release solution build completed with 0 warnings and 0 errors,
package validation produced all packages, and DocFX completed with 0 warnings
and 0 errors. NativeAOT publish and execution also completed successfully.

The ASP.NET conformance harness has one unrelated local baseline failure under
Node.js v26.5.0 in elicitation-sep1330-enums; the exact current main run
reproduces the same failure, while the remaining ASP.NET suite passes with 583
passed and 30 skipped. The repository CI workflow uses Node.js 22.

Closes #1707