test: add router and health handler tests for Phase 2 coverage

- TestRouter_HealthEndpoint: health/live/ready endpoints return 200
- TestRouter_UnknownPath: unknown paths return 404
- TestRouter_WebhookChannel_MissingChannel: empty channel returns 400
- TestRouter_WebhookPath_CanBeCalledWithGET: GET /webhook returns 405
- TestRouter_TicketsList_POST_Returns405: POST /tickets returns 405
- TestRouter_SessionsRoute_OnlyPOST: nil Sessions returns 404
- TestProbe defaults: IsLive=true, IsReady=false on NewProbe()
- TestProbe_SetLive/SetReady: atomic load/store correctness

Ref: PRODUCTION_PHASE1_STATUS.md §8.3 P1/P2 coverage gaps

internal/platform/health/health_test.go

@@ -0,0 +1,63 @@
+package health
+
+import (
+	"testing"
+)
+
+func TestProbe_IsReady_DefaultsToFalse(t *testing.T) {
+	// NewProbe sets ready to false by default
+	probe := NewProbe()
+	if got := probe.IsReady(); got != false {
+		t.Errorf("IsReady() on new probe = %v, want false", got)
+	}
+}
+
+func TestProbe_IsLive_DefaultsToTrue(t *testing.T) {
+	// NewProbe sets live to true by default
+	probe := NewProbe()
+	if got := probe.IsLive(); got != true {
+		t.Errorf("IsLive() on new probe = %v, want true", got)
+	}
+}
+
+func TestProbe_SetLive_IsLive(t *testing.T) {
+	tests := []struct {
+		name     string
+		setValue bool
+		want     bool
+	}{
+		{"SetLive(false) returns false", false, false},
+		{"SetLive(true) returns true", true, true},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			probe := NewProbe()
+			probe.SetLive(tc.setValue)
+			if got := probe.IsLive(); got != tc.want {
+				t.Errorf("IsLive() = %v, want %v", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestProbe_SetReady_IsReady(t *testing.T) {
+	tests := []struct {
+		name     string
+		setValue bool
+		want     bool
+	}{
+		{"SetReady(false) returns false", false, false},
+		{"SetReady(true) returns true", true, true},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			probe := NewProbe()
+			probe.SetReady(tc.setValue)
+			if got := probe.IsReady(); got != tc.want {
+				t.Errorf("IsReady() = %v, want %v", got, tc.want)
+			}
+		})
+	}
+}