gh-89774: Fix pprint ignoring the depth limit at small width

[tonghuaroot]

PrettyPrinter._format re-dispatches to the per-type formatter (_pprint_dict, _pprint_list, ...) whenever a node's repr exceeds the available line width. Those formatters re-expand children that _safe_repr had already folded to ... at the depth limit, so a small width silently defeats depth.

>>> import pprint
>>> pprint.pformat({1: {2: {3: 3}}}, depth=1, width=80)
'{1: {...}}'
>>> pprint.pformat({1: {2: {3: 3}}}, depth=1, width=5)
'{1: {2: {3: 3}}}'   # depth ignored

The fix skips the re-dispatch once the depth limit is reached, using the same boundary _safe_repr uses (level >= self._depth). Depth truncation is now independent of width, which also restores the narrow-width example documented in Doc/library/pprint.rst.

The regression test covers the dict, list, and tuple formatter branches and asserts that a narrow width folds at the same level as the default width. depth is always positive (__init__ rejects depth <= 0), so the is not None guard and _safe_repr's truthiness check are equivalent here.

I have left the backport labels for a maintainer to assign.

• Issue: pprint - low width overrides depth folding #89774