[AZ-809] F-AZ809-1: cap geofences.polygons at 50 (security audit)

Closes the cycle-8 Medium DoS finding. Without the cap, an
authenticated caller could submit millions of bbox polygons in a
single 500 MiB request (Kestrel global limit) and saturate the
FluentValidation allocator on the validator hot path; each polygon
is ~90 bytes of JSON, so the body limit is not a useful gate.

Realistic use is 1-10 polygons per route — 50 leaves 5x headroom
while bounding the worst-case allocation.

Layers:
- CreateRouteRequestValidator: MaxPolygons = 50 + Must(...) chained
  before RuleForEach so the count error fires at "geofences.polygons"
  (not the leaf path).
- Unit: Validate_GeofencePolygonsTooMany_FailsCountRule.
- Integration: GeofencePolygonsTooMany_Returns400 (51 valid bbox
  polygons -> HTTP 400 + errors["geofences.polygons"]).
- Contract: route-creation.md -> v1.0.1 patch (tightening an
  existing range). New Inv-10, new geofence-polygons-too-many
  test case, changelog row.
- Test spec: BT-29 sub-case 9b + AZ-809 AC-1b row in the
  traceability matrix.
- Security report: F-AZ809-1 marked RESOLVED in cycle 8; verdict
  remains PASS_WITH_WARNINGS (Lows + carry-overs unchanged).

Co-authored-by: Cursor <cursoragent@cursor.com>

SatelliteProvider.Tests/Validators/CreateRouteRequestValidatorTests.cs

@@ -249,6 +249,33 @@ public class CreateRouteRequestValidatorTests
         result.ShouldHaveValidationErrorFor("geofences.polygons");
     }
 
+    [Fact]
+    public void Validate_GeofencePolygonsTooMany_FailsCountRule()
+    {
+        // Arrange — 51 polygons; cap is 50 (security-audit F-AZ809-1 fix).
+        // Each polygon is a valid bbox so only the count rule should fire.
+        var request = ValidRequest();
+        request.Geofences = new Geofences
+        {
+            Polygons = Enumerable
+                .Range(0, 51)
+                .Select(_ => new GeofencePolygon
+                {
+                    NorthWest = new GeoPoint(50.15, 36.05),
+                    SouthEast = new GeoPoint(50.05, 36.15),
+                })
+                .ToList(),
+        };
+
+        // Act
+        var result = _validator.TestValidate(request);
+
+        // Assert — OverridePropertyName makes the count rule fire at the
+        // wire-format path `geofences.polygons` (not the leaf-only `polygons`).
+        result.ShouldHaveValidationErrorFor("geofences.polygons")
+            .WithErrorMessage("`geofences.polygons` must contain at most 50 polygons.");
+    }
+
     [Fact]
     public void Validate_CreateTilesZipWithoutRequestMaps_FailsCrossFieldRule()
     {