Address technical debt and improve safety in domain flows and models (#3065)

* Address technical debt and improve safety in domain flows and models

- Addressed unhandled empty lists and swallowed exceptions in DomainFlowTmchUtils.
- Improved null safety and immutability guarantees in Fee and LaunchPhase.
- Applied defensive copying in FeeTransformResponseExtension.
  Note: This uses the forceEmptyToNull(nullToEmptyImmutableCopy(...))
  pattern. This defensive copy ensures immutability, while forceEmptyToNull
  is required because JAXB will serialize an empty collection as an empty
  XML tag (which violates EPP XML schemas). Setting it to null ensures
  JAXB omits the tag entirely.
- Corrected JAXB property suppression in FeeCheckResponseExtensionItemStdV1.

* Add pr-polisher skill for automated PR pre-flight checks

* Enhance pr-polisher with more GEMINI.md constraints

Added checks for:
- Incorrect @Nullable imports.
- Unstatically imported utility methods (DateTimeUtils/CacheUtils).
- Redundant transaction wrapping (tm().transact -> tm().reTransact).
- Mutable collection instantiations (ArrayList/HashMap).

core/src/test/java/google/registry/flows/domain/DomainFlowTmchUtilsTest.java

@@ -0,0 +1,56 @@
+// Copyright 2026 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package google.registry.flows.domain;
+
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import com.google.common.collect.ImmutableList;
+import google.registry.flows.domain.DomainFlowTmchUtils.SignedMarksListEmptyException;
+import google.registry.flows.domain.DomainFlowTmchUtils.SignedMarksMustBeEncodedException;
+import google.registry.flows.domain.DomainFlowTmchUtils.TooManySignedMarksException;
+import google.registry.model.smd.AbstractSignedMark;
+import java.time.Instant;
+import org.junit.jupiter.api.Test;
+import org.mockito.Mockito;
+
+class DomainFlowTmchUtilsTest {
+
+  private final DomainFlowTmchUtils tmchUtils = new DomainFlowTmchUtils(null);
+
+  @Test
+  void test_verifySignedMarks_emptyList() {
+    assertThrows(
+        SignedMarksListEmptyException.class,
+        () -> tmchUtils.verifySignedMarks(ImmutableList.of(), "example", Instant.now()));
+  }
+
+  @Test
+  void test_verifySignedMarks_tooManyMarks() {
+    AbstractSignedMark mark1 = Mockito.mock(AbstractSignedMark.class);
+    AbstractSignedMark mark2 = Mockito.mock(AbstractSignedMark.class);
+    assertThrows(
+        TooManySignedMarksException.class,
+        () ->
+            tmchUtils.verifySignedMarks(ImmutableList.of(mark1, mark2), "example", Instant.now()));
+  }
+
+  @Test
+  void test_verifySignedMarks_notEncoded() {
+    AbstractSignedMark mark1 = Mockito.mock(AbstractSignedMark.class);
+    assertThrows(
+        SignedMarksMustBeEncodedException.class,
+        () -> tmchUtils.verifySignedMarks(ImmutableList.of(mark1), "example", Instant.now()));
+  }
+}