fix(cfr): fall back to Call when preflop chart prescribes raise but cap is reached

[elliottneilclark]

When max_raises_per_round (default 3) was reached during preflop,
PreflopChartActionGenerator::convert_preflop_action still emitted a
Bet(three_bet_amount) for chart entries marked Raise/ThreeBet/FourBet.
The downstream validate_actions pipeline (introduced in 598ebb5) then
stripped this bet via filter_raises_when_capped, leaving an empty
action set. ActionPicker::pick_action subsequently panicked with
"cannot sample empty range" inside rng.random_range(0..0).

The fix checks game_state.is_raise_capped() in convert_preflop_action
and returns AgentAction::Call for any raise-type chart action when the
cap is reached. This preserves the chart's intent — the hand is playable
— while respecting the raise limit so the agent calls to see the flop
instead of producing an action that gets filtered into an empty set.