fix(errs): Retry transient grouped failures

[prathshenoy]

Summary

What:

• Retry work when any one of several failures reported together is transient instead of giving up on the first attempt.
• Limit a classification attached to one grouped failure to that failure alone, instead of letting it decide the outcome for the others reported alongside it.

Why:

• Stop discarding failures that would have cleared on their own by keeping a step that fans work out to several handlers from being abandoned because one handler hit a momentary error.

Test Plan

• Add unit tests.

Revert Plan

• Revert this PR. The changes in this PR are confined to the error classification framework.

Issues

• CODEM-462

[mnoah1]

What if instead, we just create errs.Group(errors...) and let controllers return that to group errors:

• Iternal groupedError type with members[]
• when it's a grouped error, each member gets passed through Process, which gets its classification
• the whole thing gets wrapped as retryable if any of them are retryable

Would be a bit more explicit than trying to unwrap from the results returned by errors.Join