feat: check if tuples belongs to same shard before rewriting

[murex971]

fixes #1012

Adds uniform_shard() helper that iterates the split requests and returns Some(n) only if every split has Shard::Direct(n) for the same n, otherwise None.

Testing

added and ran following unit test:
cargo nextest run --profile dev -E 'test(same_shard_insert) | test(cross_shard_insert)'

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[sgrif]

It would also be nice to have a unit test for this in addition to the integration tests

[levkk]

Agreed. The integration test doesn't assert the changed behavior - it will pass in both scenarios (if the rewrite happened or not).

[sgrif]

Yeah, now that I think about it I'd just delete the integration test. This is essentially an implementation detail and not something that can be meaningfully tested at the client level

[murex971]

@sgrif @levkk removed the integration test in favour of unit tests, resolved other comments as well, thanks!

[sgrif]

Don't see anything worth blocking this over