Improve OpenAPI parsing exception logging

[AayushP123]

Summary

• Logs the OpenAPI specification path when OpenAPI parsing throws before diagnostics are available.
• Preserves the existing failure behavior by rethrowing the original exception.
• Adds regression coverage for malformed x-ms-enum-flags input.

Testing

• dotnet test ./tests/Kiota.Builder.Tests --filter OpenApiDocumentDownloadServiceTests
• dotnet build ./src/kiota --no-restore

Fixes #7014

[AayushP123]

@microsoft-github-policy-service agree

[baywet]

Thanks for the contribution!

[baywet]

Thank you for making the changes!