Add FileNotFoundError handling for missing az CLI in function deployment

[Copilot]

_deploy_functions and _deploy_functions_withslot only caught subprocess.CalledProcessError, leaving FileNotFoundError unhandled when az is not installed or not on PATH — resulting in a cryptic traceback with no guidance.

Changes

• _deploy_functions / _deploy_functions_withslot: Add FileNotFoundError catch block before CalledProcessError in both subprocess.run call sites, printing an actionable install/PATH message before re-raising.

except FileNotFoundError:
    print(
        "Error: 'az' CLI not found. Please install the Azure CLI and ensure it is on your PATH."
    )
    raise
except subprocess.CalledProcessError as e:
    print(f"Error deploying function app: {e}")
    raise

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