fix: expose on_demand_allocation_strategy to unblock instance_requirements

[dmitrijn]

instance_requirements (attribute-based instance selection) was already supported
by this module, but applying it would fail at the AWS API level with:

ValidationError: The specified OnDemandAllocationStrategy is not valid.
The prioritized allocation strategy is not compatible with instance requirements.
Valid options are [lowest-price].

This happened because on_demand_allocation_strategy was not exposed by the module,
so AWS defaulted to prioritized, which it rejects when instance_requirements is used.

Changes

• Add on_demand_allocation_strategy to runner_worker_docker_autoscaler_asg and
  pass it through to the ASG instances_distribution block
• Default to lowest-price, consistent with the existing spot_allocation_strategy default
• Add a validation that enforces lowest-price when instance_requirements is set,
  surfacing a clear terraform validate error instead of a misleading AWS 400 at apply time

Usage

To use attribute-based instance selection:

runner_worker_docker_autoscaler_asg = {
  enable_mixed_instances_policy = true
  on_demand_allocation_strategy = "lowest-price"
  instance_requirements = [{
    vcpu_count            = { min = 4, max = 16 }
    memory_mib            = { min = 16384, max = null }
    cpu_manufacturers     = ["amazon-web-services"]
    instance_generations  = ["current"]
    burstable_performance = "excluded"
  }]
}

[github-actions]

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[dmitrijn]

@npalm and @kayman-mk, could you please review this?