Expand npm conformance coverage

[halter73]

Summary

• update @modelcontextprotocol/conformance from 0.2.0-alpha.8 to 0.2.0-alpha.9
• enable offline-access and RFC 9207 issuer client scenarios now supported by the SDK
• enable json-schema-2020-12 and server-sse-polling on Windows now that the runner's completed success summary handles the known Node/libuv shutdown assertion

The alpha.9 package does not include a scenario for an initialize MCP-Protocol-Version header/body mismatch.

Intentionally not enabled

The following client scenarios still fail and require SDK functionality outside this PR: auth/authorization-server-migration, auth/client-credentials-jwt, auth/client-credentials-basic, auth/enterprise-managed-authorization, sep-2322-client-request-state, and json-schema-ref-no-deref. 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.

Validation

• dotnet build
• changed net10 conformance tests: 36 passed
• full ModelContextProtocol.AspNetCore.Tests on net8/net9/net10: 537 passed, 30 skipped per target
• direct alpha.9 probes of every previously unwired client scenario and all task-extension scenarios
• repeated Windows probes of json-schema-2020-12 and server-sse-polling

A repository-wide dotnet test --no-build run reached an unrelated environment blocker in ModelContextProtocol.Tests: stdio integration tests cannot discover TestServer.exe on this Windows machine (71-72 failures per target). All ASP.NET Core test targets, including the changed conformance coverage, passed.

[PranavSenthilnathan]

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.

Do we want to do this after this PR is merged?