IO: Consolidate PyArrow logic into io/pyarrow.py before decomposition

首先比较 table/upsert_util.py 和 pyiceberg/io/pyarrow.py，并查看 #3737 中的相关讨论。先专注于 PR A；完成的标准是 upsert utility 的 PyArrow 逻辑通过 io/pyarrow.py 路由，不改变行为，并且所有现有测试都通过。

Summary

Before decomposing pyiceberg/io/pyarrow.py into focused submodules (#3737, #3738), we should consolidate PyArrow-specific logic that currently lives outside the module. This ensures all PyArrow calls route through a single boundary, making the subsequent split clean and enabling future engine substitution.

Motivation

Per discussion in #3737, @rambleraptor noted that the first useful step is ensuring no PyArrow logic occurs outside pyarrow.py. Currently several modules import pyarrow directly and implement compute logic inline rather than delegating through pyiceberg.io.pyarrow.

When we later introduce a ComputeEngine protocol, any PyArrow logic outside the module boundary bypasses the protocol and prevents clean substitution.

Audit

Grepped pyiceberg/ (excluding io/pyarrow.py and tests) for runtime import pyarrow statements (both top-level and inline). Excluded TYPE_CHECKING-only imports since those have no runtime dependency.

Plan

One PR per absorption. Each is a pure refactor: move code into io/pyarrow.py, have the caller import from pyiceberg.io.pyarrow instead of pyarrow directly. No behavior change, all existing tests pass unchanged.

• PR A: Absorb table/upsert_util.py PyArrow logic
• PR B: table/inspect.py (pending discussion)
• PR C: transforms.py (pending discussion)

Related

• #3737 - Decompose io/pyarrow.py into focused modules
• #3738 - Extract PyArrowFileIO (first decomposition step)
• #3715 / #3716 - Previous pluggable backend attempt (rejected as too large)