refactor: Make capture overflow partitioning code a bit more straightforward

[tkaemming]

Problem

These might be personal problems, but:

1. I've been writing Python for nearly two decades and still have to look up the precedence rules for logical operator chaining like this
2. I misread the candidate_partition_key/kafka_partition_key assignments earlier (in part because my brain glossed over the complex conditional expression)

Changes

1. Made the operator precedence explicit
2. Tried to make it clearer there are two separate potential assignments for kafka_partition_key by keeping them closer together makes it glaringly obvious for the next unsuspecting reader.

Does this work well for both Cloud and self-hosted?

Yes.

How did you test this code?

Hopefully already covered, maybe?

[tkaemming]

Going to rework this more significantly to address concerns with #20945 (comment).