Fix invalid initialize request handling

[lntutor]

Summary

• validate initialize request parameters before creating an HTTP session
• return HTTP 400 with JSON-RPC InvalidParams and the original request ID
• cover a missing clientInfo.version payload

Testing

• dotnet build
• ASP.NET Core test suite on net8.0, net9.0, and net10.0

Fixes #788

[lntutor]

Rebased onto current main and resolved the StreamableHttpHandler conflict by preserving both protocol paths: July 2026 initialize requests still return the required removed-method 404, while older-protocol initialization retains invalid-parameter validation. The focused regression set passes (24 tests), and the complete ModelContextProtocol.AspNetCore.Tests project passes locally across .NET 8, 9, and 10. @PranavSenthilnathan, could you re-review when convenient?

[lntutor]

Current head 77b5d6b00a01a760bf41a3b9c3ba09fc8120142a is mergeable, the review decision is APPROVED, and the visible CI surface on this exact SHA is fully green: CodeQL, all Debug/Release build-and-test jobs on Ubuntu, macOS, and Windows, and publish-coverage all completed successfully. The current branch already carries the rebase conflict resolution and focused regression coverage noted earlier, so this looks ready for maintainer merge when convenient.