legacylibrarian: support pre-release versions

[parthea]

In https://github.com/googleapis/python-documentai-toolbox, the current version of the library is 0.15.0-alpha which is not currently supported in legacy librarian. See the error log below

(py392) partheniou@partheniou-vm-3:~/git/python-documentai-toolbox$ librarian release stage -library-version=0.16.0-alpha -library=google-cloud-documentai-toolbox
time=2025-12-18T18:56:43.793Z level=INFO msg="temporary working directory" dir=/tmp/librarian-2279678044
time=2025-12-18T18:56:43.793Z level=INFO msg="repo not specified, using current working directory as repo root" path=/usr/local/google/home/partheniou/git/python-documentai-toolbox
time=2025-12-18T18:56:43.793Z level=INFO msg="opening repository" dir=/usr/local/google/home/partheniou/git/python-documentai-toolbox
time=2025-12-18T18:56:43.964Z level=INFO msg="source not specified, skipping service config population"
time=2025-12-18T18:56:43.964Z level=ERROR msg="librarian command failed" err="failed to create stage runner: validating librarian state: invalid library at index 0: invalid version: \"0.15.0-alpha\""

The following prerelease suffixes should be supported

a
alpha
b
beta
preview
rc

With or without a leading hyphen. See https://peps.python.org/pep-0440/#pre-release-spelling for additional variations that we could support.

[julieqiu]

We are likely not supporting this in legacylibrarian, but we should make sure to support this as part of librarian.

[jskeet]

Moving to the publish milestone.

[parthea]

We are likely not supporting this in legacylibrarian, but we should make sure to support this as part of librarian.

To clarify, this functionality appears to be broken in legacylibrarian. Pre-releases can still go out, and leave the repo in a broken state. See below

• See PR chore: create a release google-cloud-python#16193 which contained version google-cloud-documentai-toolbox: v0.15.1-alpha.1 .
• The release was successfully published to PyPI as google-cloud-documentai-toolbox 0.15.1a1 https://pypi.org/project/google-cloud-documentai-toolbox/0.15.1a1/
• legacylibrarian created tag google-cloud-documentai-toolbox-v0.15.1 but it should have been google-cloud-documentai-toolbox-v0.15.1-alpha.1. The interesting part is that the contents in the release body is correct, including the v0.15.1-alpha.1 part. It's just the title and tag that's wrong.