Avoid wildcards, which confuse our nullness checker.

Specifically, wildcards defeat our ability to recognize that `containsKey` guarantees that `get` will return non-null.

See https://github.com/jspecify/nullness-checker-for-checker-framework/blob/a13e6921d6e23ec1349be007b5e9a6d1f279c91c/src/main/java/com/google/jspecify/nullness/NullSpecTransfer.java#L603

Without this CL, Auto-Common would break when I submit CL 382342656 to annotate the remaining immutable-map classes.

Arguably we should have a specific issue open for this problem, but:

- This may be the first time it's come up in practice.
- _Everything_ about wildcards in our checker is weird, thanks to a combination of the Checker Framework's unusual handling of them (which they are fixing by implementing capture conversion) and our hacks on top of that (which we hope can go away). (The very general project of "remove our hacks" is tracked by jspecify/checker-framework#4 (comment) and other issues there.)

RELNOTES=n/a
PiperOrigin-RevId: 382352859

common/src/main/java/com/google/auto/common/AnnotationMirrors.java

@@ -18,6 +18,7 @@
 import static com.google.auto.common.MoreElements.isAnnotationPresent;
 import static com.google.auto.common.MoreStreams.toImmutableSet;
 import static com.google.common.base.Preconditions.checkNotNull;
+import static java.util.Collections.unmodifiableMap;
 
 import com.google.common.base.Equivalence;
 import com.google.common.collect.ImmutableMap;
@@ -94,9 +95,10 @@ public static Equivalence<AnnotationMirror> equivalence() {
   public static ImmutableMap<ExecutableElement, AnnotationValue> getAnnotationValuesWithDefaults(
       AnnotationMirror annotation) {
     ImmutableMap.Builder<ExecutableElement, AnnotationValue> values = ImmutableMap.builder();
+    // Use unmodifiableMap to eliminate wildcards, which cause issues for our nullness checker.
     @SuppressWarnings("GetElementValues")
-    Map<? extends ExecutableElement, ? extends AnnotationValue> declaredValues =
-        annotation.getElementValues();
+    Map<ExecutableElement, AnnotationValue> declaredValues =
+        unmodifiableMap(annotation.getElementValues());
     for (ExecutableElement method :
         ElementFilter.methodsIn(annotation.getAnnotationType().asElement().getEnclosedElements())) {
       // Must iterate and put in this order, to ensure consistency in generated code.