Add support for task-extension `@modelcontextprotocol/conformance` tests

[halter73]

The alpha.9 task-extension server scenarios also remain unwired because the conformance server does not provide their required task fixtures/capabilities; tasks-status-notifications executes zero checks in that configuration.

We want to do this after #1725 is merged.

Originally posted by @PranavSenthilnathan in #1725 (comment)