Identify azd runs from Microsoft Foundry Canvas

We need a way to identify azd command runs from Microsoft Foundry Canvas.

Microsoft Foundry Canvas sets `AZURE_DEV_USER_AGENT=microsoft_foundry_canvas`. azd should append the `Microsoft Foundry Canvas` modifier to `execution.environment` while preserving the existing base environment.

Start by tracing azd's handling of execution.environment and the AZURE_DEV_USER_AGENT variable, then inspect nearby tests for environment modifiers. Done means runs initiated with microsoft_foundry_canvas retain the existing base environment and append the Microsoft Foundry Canvas modifier.

Written by the indexing model from the issue text.