Backport #118796 to 26.7: Iceberg manifest list partition summaries - #119200

`test_writes_manifest_list_partition_summary` asserts the raw bytes
of the manifest-list partition summary, and it is the first test to do
so, which is why it fails on this branch for every parametrisation
while it is green on master:

  At index 1 diff: (False, None, b'\x0b\x00\x00\x00\x00\x00\x00\x00', ...) != (False, None, b'\x0b\x00\x00\x00', ...)

The `Int32` partition bound is written as 8 bytes where Appendix D of
the Iceberg specification requires 4. `dumpValue` sizes its output as
`sizeof(T)` with `T` deduced from its argument, and
`Field::safeGet<Int32>` returns `NearestFieldType<Int32>`, which is
`Int64`, so the branch asks for an `Int32` and gets an 8-byte value.

Narrow before `dumpValue` sees it, with the same `static_cast<Int32>`
the neighbouring small-integer branch already uses. This is the source
half of `3dc6b4f4ef32` (#117571, `Closes #117072`), which never got a
backport label; this branch needs it because the test it adds pins the
spec width. That test is the coverage here, so the stateless test of
the original commit is left out rather than duplicated.

The data-file bounds are corrected by the same line, so this branch
stops writing manifests that `pyiceberg` refuses to plan a filtered
scan over (`unpack requires a buffer of 4 bytes`). No reader change is
needed in either direction: `ColumnVector::insertData` ignores the
buffer length and does `unalignedLoad<T>(pos)`, so an existing 8-byte
bound still decodes.

Related: #117571
Related: #117072