feat: add allElementsAreEqual assertion for iterables - #4330

Closes #4302

Why

There is currently no chaining-friendly way to assert that all elements of an iterable are equal to each other. The workaround assertThat(list).allMatch(e -> e.equals(list.get(0))) requires a direct reference to the first element, which is awkward when the iterable is produced by a chain of filteredOn/extracting calls and cannot be easily referenced. As discussed in the issue, @joel-costigliola agreed to add allElementsAreEqual (equality-based, not reference-based).

What

Adds allElementsAreEqual() to AbstractIterableAssert, so it is available on all Iterable-based assertions:

assertThat(dataLines).allElementsAreEqual();

• Passes when all elements are equal, and (vacuously) for empty or single-element iterables — consistent with allMatch/allSatisfy.
• Fails otherwise, reporting every element that is not equal to the first.
• Honors a custom element comparator set via usingElementComparator(...) (the assertion goes through the internal comparisonStrategy).

New error factory ShouldHaveAllElementsEqual produces a message such as:

Expecting all elements to be equal to:
  "a"
but the following element(s) were not:
  ["b"]
in:
  ["a", "b"]

Tests

• Iterables_assertAllElementsAreEqual_Test — engine behaviour: all-equal, single-element and empty pass; null actual; failure reports all unequal elements; readable error message; and the custom (case-insensitive) comparison-strategy pass/fail paths.
• IterableAssert_allElementsAreEqual_Test — verifies the public API delegates to the internal engine (IterableAssertBaseTest).

Verification done: ran ./mvnw -pl assertj-core test for both new test classes (10 tests, all green); spotless:apply and license:format produce no further changes.

I followed the discussion in #4302; if you would prefer this also exposed on ObjectEnumerableAssert (arrays) or want the pair/fold assertions tracked separately, happy to adjust.