bump_version.py drops the rc/beta suffix when bumping a dev counter on a pre-release #2459

Summary

util/bump_version.py's dev-counter bump doesn't preserve a pre-release segment: bump('1.6.0rc1.dev2') returns '1.6.0.dev3', silently dropping rc1. Under PEP 440, 1.6.0.dev3 sorts before 1.6.0rc1.dev2, so this is a version regression, not just a cosmetic loss.

Repro

from util.bump_version import bump
bump('1.6.0rc1.dev2')  # -> '1.6.0.dev3', expected something like '1.6.0rc1.dev3'

Surfaced during #2452's cross-review while checking is_devrelease/is_prerelease interactions; predates that PR and is unrelated to its gate logic.

Proposed fix

The dev-counter increment should preserve any pre-release segment already present (aN/bN/rcN) rather than reconstructing the version from just the release segment.