check that swaps involve both pool assets

[pintail-xyz]

What does this PR do?

When creating swaps, this PR adds a check that the swap involves two different assets. If the two assets in the swap are the same, then the swap is invalid and is discarded.

Related issue

327

Testing

I have verified that transaction 0xd0bc1728db3e6cbe2769c2f534a5f7a9a5a46b13428c1a3f39c18dfce0154ec1 processes successfully after applying this change.

Checklist before merging

• [x] Read the contributing guide
• [x] Installed and ran pre-commit hooks
• [x] All tests pass with ./mev test