Replace fixed test ports with free port helper

[andystaples]

Replaces fixed port numbers used by the unit tests and in-memory backend to use a shared helper that finds a free port - minimizing flaky test failures and Windows Hyper-V port conflict issues

[Copilot]

Pull request overview

This PR updates the Durable Task Python SDK test suite to stop relying on hard-coded localhost ports, instead selecting an available port at runtime via a shared helper. This reduces flaky failures caused by port collisions (notably on Windows where reserved port ranges can conflict with fixed test ports).

Changes:

• Introduced a shared find_free_port() helper for tests.
• Updated multiple E2E/integration-style tests to use dynamically selected backend ports instead of fixed values.
• Updated history-export and entity E2E tests to use the shared port helper.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 14 comments.

Show a summary per file

File	Description
tests/durabletask/test_work_item_filters_e2e.py	Switches work-item filter E2E backend to a dynamically selected port.
tests/durabletask/test_orchestration_versioning_e2e.py	Uses a dynamically selected port for orchestration versioning E2E backend.
tests/durabletask/test_orchestration_e2e.py	Uses a dynamically selected port for orchestration E2E backend.
tests/durabletask/test_orchestration_async_e2e.py	Uses a dynamically selected port for async orchestration E2E backend.
tests/durabletask/test_large_payload_e2e.py	Uses a dynamically selected port for the in-memory backend used in large-payload E2E tests.
tests/durabletask/test_batch_actions.py	Uses a dynamically selected port for batch action tests.
tests/durabletask/extensions/history_export/test_orchestrator.py	Uses a dynamically selected port for history-export orchestrator E2E backend.
tests/durabletask/extensions/history_export/test_entity.py	Uses a dynamically selected port for history-export entity E2E backend.
tests/durabletask/extensions/history_export/test_client.py	Uses a dynamically selected port for history-export client E2E backend.
tests/durabletask/extensions/history_export/test_activities.py	Uses a dynamically selected port for history-export activities E2E backend.
tests/durabletask/entities/test_function_based_entities_e2e.py	Uses a dynamically selected port for function-based entities E2E backend.
tests/durabletask/entities/test_entity_failure_handling.py	Uses a dynamically selected port for entity failure handling E2E backend.
tests/durabletask/entities/test_class_based_entities_e2e.py	Uses a dynamically selected port for class-based entities E2E backend.
tests/durabletask/_port_utils.py	Adds shared helper for choosing a free TCP port for tests.