Fix SubList.get() accepting an index equal to its own size

[renechoi]

SubList.get() checked index > size(), so an index equal to the sub list's size passed and reached AbstractImmutableList.this.get(fromIndex + index).

The issue describes this as surfacing an IndexOutOfBoundsException from the parent instead of a clear one from the sub list. That is what happens when the sub list reaches the end of its parent, and the exception is an ArrayIndexOutOfBoundsException from the backing array. When the sub list stops earlier there is no exception at all: the parent resolves the index and returns an element the sub list does not contain.

List<String> list = ImmutableCollections.copy(List.of("a", "b", "c"));
list.subList(0, 1).get(1);   // "b"   before
list.subList(0, 1).get(1);   // IndexOutOfBoundsException: Index: 1, Size: 1   after
list.subList(2, 3).get(1);   // ArrayIndexOutOfBoundsException: Index 3 out of bounds for length 3   before

ListN.get() indexes the array directly and has no bounds check of its own, which is why a read past the sub list is silent.

The same class is checked in three times: the mdo template plus the two copies generated from it under api/maven-api-xml and impl/maven-xml. All three are fixed, since changing only the copies would let the next regeneration bring it back.

listIterator(int) a few lines above keeps index > size(). That bound is correct there because List.listIterator(int) accepts size() as a cursor position, so one of the added tests pins it to prevent the two being aligned later by mistake.

Tests: four cases in a new ImmutableCollectionsTest in impl/maven-xml, covering the equal-to-size index, the read past a sub list that ends early, negative indices, and the listIterator bound. Two of them fail on maven-4.0.x and pass with this change; the module suite is green at 75 tests with checkstyle and spotless enabled.

Fixes #12595

[gnodet]

Clean, correct off-by-one bug fix in SubList.get() bounds check. The fix properly changes index > size() to index >= size() to align with the java.util.List contract, which specifies that get(size()) must throw IndexOutOfBoundsException.

Key observations:

• All three copies of the file are fixed (template + both generated copies), preventing reintroduction on code generation
• The listIterator(int index) method correctly retains index > size() since List.listIterator(size()) is a valid cursor position
• The four new test cases are well-chosen: they reproduce both bug variants, verify normal iteration, and pin the listIterator contract
• This is the first test class for ImmutableCollections, adding coverage to a previously untested area

This review was generated by an AI agent and may contain inaccuracies. Please verify all suggestions before applying.

Claude Code on behalf of gnodet