Packaging Test fails: 'Invalid distribution metadata: 2.5 is not a valid metadata version'

`check_packaging` fails on every PR right now, unrelated to the PR contents.

```
Checking dist/transformerbeeclient-0.1.dev1+g47ac9e3f6-py3-none-any.whl:
ERROR InvalidDistribution: Invalid distribution metadata: '2.5' is not a valid metadata version
```

The build backend now emits `Metadata-Version: 2.5`, which the `twine` version resolved in the test env does not recognise. Nothing in the repo changed to cause it — the job last passed on 2026-08-12 (`migrate-to-ruff`) and fails on runs since, so it is dependency drift in an unpinned tool, not a code regression.

Fix is presumably to pin/raise `twine` (>= 6.2 recognises metadata 2.5) in the packaging test environment.

Found while working on #210, where this is the only failure not attributable to that PR.

*Filed with Claude Code.*

Start by locating the check_packaging job and its packaging test environment, then inspect how twine is resolved. Ensure the environment uses twine >= 6.2 and rerun check_packaging; done means the wheel with Metadata-Version 2.5 passes validation.

Written by the indexing model from the issue text.