Upgrade MCP C# SDK to v1.2.0 and re-enable InMemoryMcpTaskStore

[jongalloway]

Summary

Upgrades ModelContextProtocol from v1.1.0 → v1.2.0 and re-enables InMemoryMcpTaskStore, which was disabled due to csharp-sdk#1430. The upstream fix shipped in v1.2.0 via csharp-sdk#1433.

Fixes #410

Changes

File	Change
DotNetMcp/DotNetMcp.csproj	ModelContextProtocol 1.1.0 → 1.2.0
DotNetMcp.Tests/DotNetMcp.Tests.csproj	ModelContextProtocol 1.1.0 → 1.2.0
DotNetMcp/Program.cs	Uncomment AddSingleton<IMcpTaskStore, InMemoryMcpTaskStore>()
DotNetMcp/Tools/Cli/DotNetCliTools.Misc.cs	Remove stale bug-reference comment on AsyncTasks
DotNetMcp.Tests/Server/ServerCapabilitiesTests.cs	Pass InMemoryMcpTaskStore to test fixture; flip AsyncTasks assertions to true
DotNetMcp.Tests/Server/McpConformanceTests.cs	Replace graceful-degradation tests with task lifecycle conformance tests
README.md	Update MCP C# SDK docs URL to new vanity domain

Checklist from #410

• Upstream issue fixed and new MCP SDK version released (v1.2.0, 2026-03-27)
• Uncomment builder.Services.AddSingleton<IMcpTaskStore, InMemoryMcpTaskStore>() in Program.cs
• AsyncTasks in DotNetCliTools.Misc.cs — already derived from _taskStore != null, now resolves to true
• Restore task lifecycle conformance tests in McpConformanceTests.cs
• Update ServerCapabilitiesTests.cs to assert AsyncTasks == true

v1.2.0 impact assessment

v1.2.0 Change	Relevance
DI scope fix (#1433)	Direct fix — enables InMemoryMcpTaskStore
WithMeta + WithProgress fix (#1464)	Silent fix — 4+ consolidated tools use both; working correctly now
Obsolete 2-arg RequestContext (MCP9003)	Not used in codebase
Legacy SSE disabled (#1468)	N/A — stdio-only server
Outgoing message filter routing (#1465)	No outgoing filters used
OutputSchema for CallToolResult (#1425)	Filed as #423 for future work
Docs vanity domain (#1414)	Fixed stale README URL

Test results

Test run summary: Passed!
  total: 1366
  failed: 0
  succeeded: 1320
  skipped: 46
  duration: 7m 00s