fix(ray): name the missing flag instead of failing on int + None

[Arist12]

Problem

Without --colocate the rollout GPU count cannot be derived, and leaving --rollout-num-gpus unset surfaces several frames later as

TypeError: unsupported operand type(s) for +: 'int' and 'NoneType'

from placement-group sizing, which never names the flag. --debug-rollout-only has the same gap one step earlier, in miles_validate_args, where the count feeds min(8, ...).

Reproduced by dropping --rollout-num-gpus 1 from examples/lora/run-qwen2.5-3B-megatron-lora-disaggregated.sh.

Fix

Assert at each point of use, naming the flag. Both places, because they are reached by different paths — _get_placement_group_layout for the ordinary disaggregated run, miles_validate_args for --debug-rollout-only, which fails before the placement group is ever built.

Not in miles_validate_args for the general case: about 20 tests in tests/fast/utils/test_arguments.py call it with a minimal argument set (colocate off, rollout_num_gpus unset) and expect it to pass.

Test

One case added to tests/fast/ray/test_placement_group_shared_ppo.py, which already covers _get_placement_group_layout for colocate / debug_train_only / rollout_external with the same fixture.