Add credential validation for App Insights connection retrieval

[Copilot]

Addresses missing null and empty checks when accessing Application Insights connection credentials. The Key field is marked omitempty in the JSON struct, making it potentially absent from API responses.

Changes

• Validate credentials before use: Check Credentials.Key is non-empty before setting APPLICATIONINSIGHTS_CONNECTION_STRING environment variable
• Fail fast on prompt errors: Return error immediately when connection selection prompt fails, rather than continuing with nil connection

Example

// Before: Could panic or set empty connection string
if err := a.setEnvVar(ctx, "APPLICATIONINSIGHTS_CONNECTION_STRING", selectedConnection.Credentials.Key); err != nil {
    return err
}

// After: Validate before use
if selectedConnection.Credentials.Key == "" {
    return fmt.Errorf("Application Insights connection '%s' has no credentials key", selectedConnection.Name)
}
if err := a.setEnvVar(ctx, "APPLICATIONINSIGHTS_CONNECTION_STRING", selectedConnection.Credentials.Key); err != nil {
    return err
}

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