fix(testing): scale local durable timers

[zhongkechen]

Summary

• Add a shared DURABLE_EXECUTION_TIME_SCALE helper for local durable test timers.
• Apply the scale to wait timers, step retry timers, callback timeouts, and callback heartbeat timeouts.
• Default local example tests to 0.05 scale so modeled durable delays do not spend unnecessary wall-clock time in CI.
• Keep a 5s floor for long callback timeout/heartbeat timers so externally-driven callback tests do not race unrealistically short deadlines.
• Let heartbeat-focused examples opt out with time_scale="1.0" because they intentionally test real heartbeat timing.

Context

These commits were split out from PR #498. They are not part of the root-cause fix for the scheduler hang or the Python 3.13/3.14 callback race; those remain in PR #498.

This PR keeps the timer scaling change separate so it can be reviewed as a test-duration improvement for local durable timers.

Changes

• Add aws_durable_execution_sdk_python_testing.time_scale.
• Reuse the helper in wait and step retry processors instead of parsing DURABLE_EXECUTION_TIME_SCALE inline.
• Scale callback timeout and heartbeat scheduling in the local test executor.
• Preserve short callback deadlines by using max(scaled_delay, min(original_delay, 5s)) for long callback timers.
• Add test coverage for scaled wait, step retry, callback timeout, callback heartbeat, and short-deadline preservation.

Verification

• hatch fmt --check
• hatch run types:check
• git diff --check
• hatch run test:all packages/aws-durable-execution-sdk-python-testing/tests/checkpoint/processors/wait_test.py packages/aws-durable-execution-sdk-python-testing/tests/checkpoint/processors/step_test.py packages/aws-durable-execution-sdk-python-testing/tests/executor_test.py -q - 138 passed
• hatch run test:all packages/aws-durable-execution-sdk-python-examples/test/callback/test_callback_heartbeat.py packages/aws-durable-execution-sdk-python-examples/test/wait_for_callback/test_wait_for_callback_heartbeat.py -q - 2 passed

Note: the heartbeat example run still shows the existing asyncio.iscoroutinefunction deprecation warning from main; that warning is fixed separately in PR #498.

[SilanHe]

I'm not against this change since you're already done the work but barring more comments, I think it would be better to just port the "skip time" functionality from the other testing libraries

[zhongkechen]

I'm not against this change since you're already done the work but barring more comments, I think it would be better to just port the "skip time" functionality from the other testing libraries

This is different from Java as the way python test cases interact with the execution is different. In Java local test runner, the runner stops execution after each invocation and waits for user interaction while pythons runner runs execution concurrently when user interacts with the execution.