chore(ic): add flexible message ordering to StateMachine tests

[venkkatesh-sekar]

Adds flexible message ordering to StateMachine for deterministic control over which canister executes which message in each round.

How it works

• StateMachineBuilder::new().with_flexible_ordering().build() configures scheduler_cores=1, tight round/subnet instruction budgets (1 message per round), and wraps the scheduler with FlexibleOrderingScheduler (priority
  boosting).
• buffer_ingress_as() buffers ingress messages, execute_with_ordering(MessageOrdering(...)) steps through them.
• Each step ticks until the message is consumed, handling DTS slices, heartbeats, timers, and management canister loopback pipelines automatically via has_in_flight_work progress detection.

Production code changes

• SchedulerConfig: added subnet_messages_per_round_instruction_limit field, moved SUBNET_MESSAGES_LIMIT_FRACTION to config as pub.
• round_schedule.rs: relaxed 3 debug assertions to tolerate scheduler_cores=1.
• scheduler.rs: uses config field for subnet instruction limit instead of hardcoded formula.

What's NOT covered

• Multi-subnet / XNet: single subnet only.
• Heartbeat/Timer as explicit ordering variants: they execute implicitly before input queue messages. The design doc mentions Heartbeat(CanisterId) / Timer(CanisterId) variants — not implemented.