refactor(inline): Remove Dummy Popup Creation in Prefetch Flow and Ensure Session Validation

[evanliu048]

Description

This change removes the unnecessary creation of a dummy popup (JBPopup) in the CodeWhisperer prefetch flow. Instead, the popup field in InvocationContext is now nullable (JBPopup? = null). This ensures that prefetch logic does not create unused UI elements while maintaining compatibility with existing workflows.
reference: #5290

Additionally, it introduces session ID validation before prefetching recommendations, ensuring that only relevant recommendations are fetched.

Checklist

• My code follows the code style of this project
• I have added tests to cover my changes
• A short description of the change has been added to the CHANGELOG if the change is customer-facing in the IDE.
• I have added metrics for my changes (if required)

License

I confirm that my contribution is made under the terms of the Apache 2.0 license.

[andrewyuq]

I feel like it's ok to just create the another popup at nextInvocaationContext creation time and don't use it until when promoteNextInvocationIfAvailable is called