Add In-Process backend for E2E testing

[andystaples]

Adds a fully-featured in-memory Durable backend for E2E testing. Removes the previous dependency on the durabletask-go sidecar. Implement missing tests for durabletask including tests for batch actions, entities, and versioning.

[Copilot]

Pull request overview

This PR introduces a new in-process, in-memory gRPC backend (durabletask.testing) intended to replace the external durabletask-go sidecar dependency for core durabletask end-to-end tests, and updates CI/docs accordingly.

Changes:

• Added InMemoryOrchestrationBackend plus create_test_backend() factory under durabletask/testing/.
• Updated/added core tests/durabletask/* E2E-style tests to run against the in-process backend (including batch actions and entity scenarios).
• Simplified developer + CI test execution (Makefile/docs/workflow) by removing sidecar setup.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
tests/durabletask/test_orchestration_versioning_e2e.py	Switches versioning E2E test to in-process backend and explicit host/port.
tests/durabletask/test_orchestration_e2e.py	Switches orchestration E2E tests to in-process backend and explicit host/port; adjusts suspend waiting.
tests/durabletask/test_in_memory_backend_entities.py	Adds entity-focused tests against the in-memory backend.
tests/durabletask/test_in_memory_backend.py	Adds basic orchestration/activity/timer/sub-orchestration tests against the in-memory backend.
tests/durabletask/test_batch_actions.py	Implements batch query/purge/entity query tests using the in-memory backend.
tests/durabletask/entities/test_function_based_entities_e2e.py	Adds function-based entity E2E tests using the in-memory backend.
tests/durabletask/entities/test_entity_failure_handling.py	Adds entity failure-handling E2E tests using the in-memory backend.
tests/durabletask/entities/test_class_based_entities_e2e.py	Adds class-based entity E2E tests using the in-memory backend.
tests/durabletask/entities/init.py	Marks the new entities test package.
pyproject.toml	Removes the prior e2e marker configuration.
examples/in_memory_backend_example.py	Adds a runnable example showing how to use the in-memory backend.
durabletask/testing/in_memory_backend.py	Implements the in-memory gRPC backend and create_test_backend() helper.
durabletask/testing/init.py	Exposes InMemoryOrchestrationBackend and create_test_backend as public testing utilities.
durabletask/testing/README.md	Documents testing utilities and the in-memory backend usage.
docs/development.md	Updates dev instructions to run tests without a sidecar.
Makefile	Collapses test-unit/test-e2e into a single test target.
.github/workflows/durabletask.yml	Removes Go sidecar installation and runs core tests without sidecar.