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

[PranavSenthilnathan]

Missing params or "params": null can deserialize to null without throwing, so this still proceeds to GetOrCreateSessionAsync, creates a session, and leaves the original observable side effect in place. Could we treat a null result as InvalidParams too and add test coverage for both missing and explicit-null params?

Note

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