.Net Agents - Fix polling cycle to properly evaluate failure mode on exception

[crickman]

Motivation and Context

Assistant polling not propertly responding to terminal error conditions.

Fixes: #9579

Description

• Added explicit check for cancelling state: Invoke, InvokeStreaming, & Polling loops
• Discriminate on different exception modes when polling
• Ensure stale run state isn't evaluated in polling loop
• Remove initial polling delay

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 😄