[CoR]No validation for invalid subscription input from chat during deployment planning

OS: Win11 x64
VS Code Insider: 1.132.0-insider
Build Version: 0.12.7-alpha.0
Regression: Not a regression

Repro Steps:

1. Start a deployment workflow
2. When prompted for the Azure subscription in chat, enter an arbitrary invalid value (for example, "test-subscription")
3. Continue through the deployment planning process
4. Approve the deployment plan and proceed with deployment

Expect:
The workflow should validate the subscription input and reject invalid values

Actual:
The invalid subscription input is accepted and the workflow continues without any validation or error message.

More Info:

1. After entering "test-subscription", the value appeared as the first option and selected value in the Subscription dropdown on the Deployment Plan page.
2. During deployment, the workflow used the current Azure subscription (AzCode2605) instead of the value entered by the user.
3. No validation message was shown and the workflow did not indicate that the entered subscription value was invalid.

Reproduce the deployment workflow and enter an invalid value such as "test-subscription" when chat requests the Azure subscription. Trace that value through the Deployment Plan page and deployment flow, then verify that invalid subscriptions are rejected with an error and cannot be used to continue deployment.