Bump ModelContextProtocol 1.4.0 -> 2.0.x in both projects

[jongalloway]

Bump ModelContextProtocol 1.4.0 → 2.0.x

Part of #481. Blocked by the governance-package gating issue (#SUB1).

Update the package reference in both projects and resolve the resulting compile breaks (tracked in sibling issues).

Tasks

• Update ModelContextProtocol to 2.0.x in DotNetMcp/DotNetMcp.csproj.
• Update ModelContextProtocol to 2.0.x in DotNetMcp.Tests/DotNetMcp.Tests.csproj.
• Restore/build and triage the compiler errors and MCP9005/MCPEXP* diagnostics into the sibling issues.
• Confirm stdio transport startup still works end-to-end (server starts, tools/list responds).

Notes

• stdio transport is structurally unaffected by the 2.0 HTTP/statelessness changes; protocol-version negotiation is handled by the SDK.

Acceptance

• Both projects reference SDK 2.0.x and the solution builds (after sibling fixes land).