azure.ai.agents - Introduce credential helper to validate auth issues up front

[JeffreyCA]

Problem

Users in multi-tenant Azure environments can sometimes see a misleading error when deploying agents:

AzureDeveloperCLICredential: please run "azd auth login" from a command prompt to authenticate before using this credential

This occurs despite being logged in, because the Azure SDK's AzureDeveloperCLICredential broadly matches any error containing "azd auth login" and replaces it with a generic message.

Solution

Added early credential validation via a new azure.NewCredential() helper that:

• Creates the credential with the correct tenant configuration
• Immediately validates by calling GetToken() (token is cached for reuse)
• Returns actionable error guidance when validation fails

New error message:

failed to authenticate for subscription '<sub>' in tenant '<tenant>'.
Suggestion: Visit this subscription in Azure Portal, then run `azd auth login`