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 general server-lifetime cancellation and IAsyncDisposable
  registration seam for extension infrastructure, with an unregistration
  handle so completed work is not retained.
• Cancel the lifetime at the start of stateful server disposal, close
  registration after request handlers drain, and await the remaining runners.
• Let each Tasks runner state own cancellation and completion awaiting, while
  long-lived MRTR handlers remain linked to the server lifetime and completed
  registrations are removed.
• 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 current lifetime-registration focus passed on net10.0, net9.0, and net8.0
(5 tests on each framework). The merged net10.0 lifecycle and Tasks
execution-mode focus passed 61 tests. The full Core suite passed with 2328 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.

Closes #1707

[jstar0]

Rebased onto current main and refreshed the lifecycle tests for the current request-filter API.

Local verification after the rebase:

• focused lifecycle tests: 5/5 passed on net10.0
• full ModelContextProtocol.Tests suite: passed on net10.0
• make build CONFIGURATION=Release: passed with 0 warnings and 0 errors

The new hosted run is in progress. The earlier Windows Release failure was in the unrelated OAuth metadata fallback test (duplicate fallback requests), not in the files changed here.

[jstar0]

The Windows Release job for head db7f5c3042103defe06c3707fe5e6898cee75952 completed with two failures outside the files changed in this PR:

• RawHttpConformanceTests.July2026Post_MissingRequiredCapability_Returns400 (net8.0): expected BadRequest, actual OK.
• OAuth.TokenCacheTests.GetTokenAsync_InvalidCachedTokenTriggersAuthDelegate: failed to find authorization-server metadata for https://localhost:7029/.

Windows Debug, Ubuntu Debug/Release, and macOS Debug/Release passed. Could you please rerun the Windows Release job or confirm whether these failures reproduce on the base branch? The lifetime/task-runner changes in this PR do not touch the ASP.NET conformance or OAuth test paths.