Mark the azure-functions-durable package as typed (PEP 561)

[andystaples]

Upstream report: Azure/azure-functions-durable-python#524

The azure-functions-durable v2 distribution should participate in PEP 561 type discovery. Strict Pyright currently reports a missing type-stub or typed-package diagnostic for azure.durable_functions.

Acceptance criteria

• Add a py.typed marker to the azure.durable_functions package.
• Ensure the marker is included in wheels and source distributions.
• Add a packaging/type-check smoke test that installs the built artifact and imports azure.durable_functions under strict Pyright or the repository's existing type checker.
• Address concrete annotation errors exposed by enabling PEP 561 rather than suppressing them globally.