Prevent generator syncs from regressing the README version

Summary

The generator sync in python-sdk#307 regressed the README version.

The SDK README version must not be overwritten with an older generator-managed value.

Required changes

• Define the owner of the Python SDK README version.
• Prefer a release-please-managed SDK version when that is supported.
• Prevent generator syncs from regressing the README version.
• Define the process for README-only whitespace changes separately from version synchronization.

Related work

Generator tracking issue: to be linked after creation.

References

• Pull request: https://github.com/openfga/python-sdk/pull/307
• Review: https://github.com/openfga/python-sdk/pull/307#pullrequestreview-4843061118
• Requested by: @SoulPancake

Acceptance criteria

• A generator sync cannot downgrade or otherwise regress the README version.
• The version update process has one documented source of truth.
• The implementation or workflow has automated coverage for the regression case.