build(deps): update ModelContextProtocol to 2.1

[kaspermarstal]

Summary

• update ModelContextProtocol from 1.2.0 to 2.1.0
• refresh the application and test lock files
• retain the existing client integration after reviewing the official 2.0 and 2.1 migration notes; Cellm does not use the removed or changed server-only APIs

The 2.x client now uses discovery-first negotiation and includes improved HTTP transport fallback and status handling. No Cellm source changes were required.

Validation

• dotnet restore Cellm.sln --locked-mode
• dotnet build Cellm.sln --no-restore — 0 warnings, 0 errors
• dotnet test Cellm.sln --filter "FullyQualifiedName~Cellm.Tests.Unit" --no-build --verbosity minimal — 86 passed
• dotnet format Cellm.sln --verify-no-changes --no-restore
• dotnet list Cellm.sln package --vulnerable --include-transitive
• real stdio smoke test against @playwright/mcp@latest — connected and discovered 25 tools

The vulnerability scan continues to report the pre-existing transitive MessagePack 2.3.85 advisories in Cellm.Tests; this update does not change them.

References

• https://github.com/modelcontextprotocol/csharp-sdk/releases/tag/v2.0.0
• https://github.com/modelcontextprotocol/csharp-sdk/releases/tag/v2.1.0