test(P0-4): 补齐 health platform 测试 - Evaluate 覆盖 100%

扩展 internal/platform/health/health_test.go（+5 个测试）:
- TestEvaluate_NoCheckers_ReturnsTrue: nil checkers → true
- TestEvaluate_EmptyCheckers_ReturnsTrue: empty checkers → true
- TestEvaluate_AllCheckersPass_ReturnsTrue: 全通过 → true
- TestEvaluate_SomeCheckersFail_ReturnsFalse: 部分失败 → false
- TestEvaluate_NilChecker_Skipped: nil checker 跳过不 panic
- TestEvaluate_AllCheckersFail_ReturnsFalse: 全失败 → false

**覆盖率提升**:
- internal/platform/health: 38.1% → **100.0%** (+61.9%) ✅
- 整体覆盖率: 73.6% → **74.8%** (+1.2%)

Ref: test/PHASE2_TEST_PLAN.md P0-4

projects/ai-customer-service/internal/platform/health/health_test.go

@@ -1,6 +1,8 @@
 package health
 
 import (
+	"context"
+	"errors"
 	"testing"
 )
 
@@ -61,3 +63,114 @@ func TestProbe_SetReady_IsReady(t *testing.T) {
 		})
 	}
 }
+
+func TestEvaluate_NoCheckers_ReturnsTrue(t *testing.T) {
+	ctx := context.Background()
+	healthy, results := Evaluate(ctx, nil)
+	if !healthy {
+		t.Errorf("Evaluate(nil) healthy = %v, want true", healthy)
+	}
+	if results != nil {
+		t.Errorf("Evaluate(nil) results = %v, want nil", results)
+	}
+}
+
+func TestEvaluate_EmptyCheckers_ReturnsTrue(t *testing.T) {
+	ctx := context.Background()
+	healthy, results := Evaluate(ctx, []Checker{})
+	if !healthy {
+		t.Errorf("Evaluate([]) healthy = %v, want true", healthy)
+	}
+	if results != nil {
+		t.Errorf("Evaluate([]) results = %v, want nil", results)
+	}
+}
+
+func TestEvaluate_AllCheckersPass_ReturnsTrue(t *testing.T) {
+	ctx := context.Background()
+	checkers := []Checker{
+		stubChecker{name: "db", err: nil},
+		stubChecker{name: "redis", err: nil},
+	}
+	healthy, results := Evaluate(ctx, checkers)
+	if !healthy {
+		t.Errorf("Evaluate() healthy = %v, want true", healthy)
+	}
+	if len(results) != 2 {
+		t.Errorf("len(results) = %d, want 2", len(results))
+	}
+	for _, r := range results {
+		if r.Status != "UP" {
+			t.Errorf("result %s status = %s, want UP", r.Name, r.Status)
+		}
+	}
+}
+
+func TestEvaluate_SomeCheckersFail_ReturnsFalse(t *testing.T) {
+	ctx := context.Background()
+	checkers := []Checker{
+		stubChecker{name: "db", err: nil},
+		stubChecker{name: "redis", err: errors.New("connection refused")},
+	}
+	healthy, results := Evaluate(ctx, checkers)
+	if healthy {
+		t.Errorf("Evaluate() healthy = %v, want false", healthy)
+	}
+	if len(results) != 2 {
+		t.Errorf("len(results) = %d, want 2", len(results))
+	}
+	for _, r := range results {
+		if r.Name == "redis" && r.Status != "DOWN" {
+			t.Errorf("redis result status = %s, want DOWN", r.Status)
+		}
+		if r.Name == "db" && r.Status != "UP" {
+			t.Errorf("db result status = %s, want UP", r.Status)
+		}
+	}
+}
+
+func TestEvaluate_NilChecker_Skipped(t *testing.T) {
+	ctx := context.Background()
+	checkers := []Checker{
+		stubChecker{name: "db", err: nil},
+		nil,
+		stubChecker{name: "cache", err: nil},
+	}
+	healthy, results := Evaluate(ctx, checkers)
+	if !healthy {
+		t.Errorf("Evaluate() healthy = %v, want true (nil skipped)", healthy)
+	}
+	if len(results) != 2 {
+		t.Errorf("len(results) = %d, want 2 (nil skipped)", len(results))
+	}
+}
+
+func TestEvaluate_AllCheckersFail_ReturnsFalse(t *testing.T) {
+	ctx := context.Background()
+	checkers := []Checker{
+		stubChecker{name: "db", err: errors.New("db down")},
+		stubChecker{name: "redis", err: errors.New("redis down")},
+	}
+	healthy, results := Evaluate(ctx, checkers)
+	if healthy {
+		t.Errorf("Evaluate() healthy = %v, want false", healthy)
+	}
+	if len(results) != 2 {
+		t.Errorf("len(results) = %d, want 2", len(results))
+	}
+	for _, r := range results {
+		if r.Status != "DOWN" {
+			t.Errorf("result %s status = %s, want DOWN", r.Name, r.Status)
+		}
+	}
+}
+
+// stubChecker is a test double for Checker interface.
+type stubChecker struct {
+	name string
+	err  error
+}
+
+func (s stubChecker) Name() string { return s.name }
+
+func (s stubChecker) Check(_ context.Context) error { return s.err }