Fix BlazorGateway VS F5 launch (#66836) and E2E manifest task path (#66617) (#66849)

* Fix BlazorGateway VS F5 launch and E2E manifest task path

- Fix #66836: Convert BlazorGateway RunCommand/RunArguments from target-based
  to static properties so VS can read them during F5/Ctrl+F5 launch (matching
  the old DevServer pattern).

- Fix #66617: Update _E2ETasksAssembly path probe in
  Microsoft.AspNetCore.Components.Testing.targets to walk up two directories
  from buildTransitive/net10.0/ to the package root tasks/ folder. Also add a
  build warning when E2E app projects are declared but the task assembly is
  missing.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Switch Components Testing task manifest serialization to DataContractJsonSerializer

Co-authored-by: javiercn <6995051+javiercn@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>