Summary

dstack offer without --group-by uses the same /runs/get_plan path as normal run planning, but still ran best-fleet selection before overriding the result with the offer-specific collection path.

This meant dstack offer without --fleet still did candidate-fleet selection and find_optimal_fleet_with_offers(...), then threw that work away and returned global offers.

Also updated in this change:

• get_job_plans() description to explain dstack offer without --group-by vs normal planning
• _should_select_best_fleet_candidate() naming and description to make the contract clearer
• a TODO next to the current commands == [":"] hack to replace it with an explicit request/API signal

Reproduce

1. Create several fleets in a project.
2. Run dstack offer --gpu L4.
3. Check server logs.

Before this change, the request evaluated candidate fleets and called find_optimal_fleet_with_offers(...) even though the final result came from the dstack offer without --group-by path.