feat(testing): add skip-time virtual clock

[yaythomas]

Description of changes:

• Add Clock protocol with RealClock and SkipClock implementations
• Give each ExecutionWorker its own clock via the registry factory
• Drive wait and step-retry timers off the clock so modeled delays complete instantly under skip while history keeps real durations
• Default DurableFunctionTestRunner to skip_time=True; pass skip_time=False for real wall-clock timing
• Remove the DURABLE_EXECUTION_TIME_SCALE env-var scaling

Issue #, if available:
closes #437

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[zhongkechen]

LGTM

The base branch was changed.

[yaythomas]

rebased on main, no other change

[edit:] ooh hang on CI failures 😕 , investigating

failing tests, fix inbound

[zhongkechen]

seems wait_for_callback(...) failed before the test ever sends a callback.

[yaythomas]

changed:

1. Advancing SkipClock: now() = real time + accumulated skip, arm() adds the gap — monotonic, distinct timestamps, matching fake-timers shouldAdvanceTime: true.
2. Per-runner clock: registry and worker are clock-free again; one Clock is shared by the Executor, CheckpointProcessor, and both runner types.
3. All history stamp sites moved to the clock: execution start/end, wait and callback completions, invocation start/end, the history fallback, and the stop response now reuses the stamped time. events_to_operations uses the event's own timestamp instead of wall-clock. Test-only fallbacks go through real_now().
4. TID251 ruff ban on datetime.now outside clock.py — verified it fires on a synthetic violation.

Verification: 1353 testing-package tests pass, full workspace run 2883 passed, the 4 previously failing callback tests pass locally and in CI, mypy clean (119 files), hatch fmt --check clean.

3.13 integration test failure failing only on test_otel_logger_example_spans_in_xray, which is known pre-existing OTel X-Ray flk.