feat(testing): invoker state and checkpoint stamping

[yaythomas]

What

Align the local test runner's data-plane responses with the real service, validated against the JS example suite as a cross-language conformance check.

Changes

Invoker state and checkpoint stamping:

• Track invocation state (PRE_INVOKE/INVOKING/COMPLETED) and gate checkpoints so at most one handler invocation is in flight per execution.
• Record per-update timestamps and stamp checkpoint operations with a monotonic sequence so delta computation and pagination share one snapshot.
• Offload handler invocation off the scheduler event loop so concurrent map/parallel branches no longer serialize on a single thread.
• Return a locked snapshot from in-process state reads.

History generation (GetDurableExecutionHistory):

• Emit one history event per checkpoint update so step retries report the correct StepStarted/StepFailed counts and attempt numbers.
• Reconstruct events from recorded update transitions using the real operation for details, with a fallback for operations completed outside the checkpoint path.

Fidelity fixes:

• Store the RETRY payload as the step result so waitForCondition state replays, and map a RETRY without error to StepSucceeded.
• Set callback timeout errorType to Callback.Timeout/Callback.Heartbeat with the correct message strings.
• Introduce CheckpointCore.match_cached as the single source of truth for idempotent checkpoint replay detection (both web and in-process paths delegate to it).

Invocation timeout:

• Add execution_timeout and invocation_timeout to the runner constructor and CLI. A timed-out invocation leaves the step STARTED and replays.

Testing

• 1311 passed, 95.67% coverage (gate 95%)
• Typecheck clean (102 files)
• Format clean
• Cross-language conformance: JS example suite failures reduced from 27 to 7 (remaining are CHAINED_INVOKE feature gap and one upstream JS SDK bug)

Stack

This is PR 1 of 2. PR 2 (per-execution worker serialization #516) builds on top.

Closes #464, #460, #462, #438, #439

[yaythomas]

ooof, this belongs elsewhere.

[yaythomas]

moved to CheckpointValidator