Turn on `-Werror` in our unit tests

[Zsailer]

@bollwyvl mentioned that we should turn on -Werror in our unit tests. Opening this issue to track here.

There are warnings that are being ignore today, that we should address.

One point raised in the meeting was that DeprecationWarnings can create a lot of noise here, making it difficult to use this flag. We should discuss how to address this.

[bollwyvl]

Yep: there are challenges.

To echo my response to @vidartf 's point: doing this without also have a {uv|pixi|poetry-shudder}.lock means PRs will fail for things for which the contributor did not change, and create a lot of noise.

One approach to this is to:

• use one of those tools to get to a checked-in lockfile
  • test against this on the happy path/big matrix
    • also improves sanity of caching
  • if a PR changes deps
    • it may also have to update deprecation warnings
• on some cadence (nightly, merges to main, magic commit strings), in dedicated CI workflow:
  • delete the lockfile
  • run the tests
  • report the diff of the lockfile and the new errors