.Net: Handle missing operation id in OpenApi spec

[markwallace-microsoft]

Motivation and Context

Closes #7343 Closes #5598

Description

Contribution Checklist

• [ ] The code builds clean without any errors or warnings
• [ ] The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• [ ] All unit tests pass, and I have added new tests where possible
• [ ] I didn't break anyone :smile: