[gty404]

Summary

• nanoarrow FULL validation only requires a struct's children to be at least as long as the parent struct, but ParseManifestList, ParseDataFile, and ParseManifestEntry index containers that are pre-sized to the parent's row count using each child's own length. A spec-legal batch with a longer child drives those loops past the end of the output vector.
• Add AssertChildrenLength, which asserts every child of a struct view has exactly the same length as the parent, and call it everywhere a struct's children feed a parent-sized container.
• The kPartitionFieldId case in ParseDataFile previously validated the partition struct's type with a bare AssertViewType (no children-count check at all); it now uses AssertViewTypeAndChildren plus the new length check, matching the other call sites.

Test plan

• Added AssertChildrenLengthTest.RejectsChildLongerThanParent in manifest_reader_test.cc, hand-building an Arrow StructArray whose declared length is shorter than its child's actual length (legal under nanoarrow FULL validation) and asserting InvalidArgument.
• Confirmed the new test fails without the fix (temporarily neutered AssertChildrenLength to a no-op) and passes with it restored.
• Ran the full manifest_test suite: 227 passed, 0 failed.