[Intelligent Router] T08: Implement priority-only strategy

[leocamello]

Overview

Always select lowest priority number.

Estimated Time: 1 hour
Dependencies: T03 (#74)

Tests to Write First

#[test]
fn selects_lowest_priority_number() {
    let candidates = vec![
        create_backend("a", 5),   // priority 5
        create_backend("b", 1),   // priority 1 (lowest)
        create_backend("c", 10),  // priority 10
    ];
    let router = create_router_with_strategy(RoutingStrategy::PriorityOnly);
    let selected = router.select_with_priority_only(&candidates);
    assert_eq!(selected.name, "b");
}

#[test]
fn selects_first_on_priority_tie() {
    let candidates = vec![
        create_backend("a", 1),
        create_backend("b", 1),
    ];
    let router = create_router_with_strategy(RoutingStrategy::PriorityOnly);
    let selected = router.select_with_priority_only(&candidates);
    assert_eq!(selected.name, "a");
}

Implementation Steps

1. Implement select_with_priority_only()
2. Find minimum priority value
3. Return first backend with that priority

Acceptance Criteria

• Selects backend with lowest priority number
• Returns first on priority tie (stable)
• Ignores load and latency

References

• Spec
• Plan
• Tasks