chore: Fix 1.87.0 clippy lints

[loewenheim]

The only potentially controversial change is the result_large_err one:

• In attrs.rs I opted for an #allow, although the functions could be changed to not return Self in the Err case because it's never used.
• In managed_envelope.rs I boxed the ManagedEnvelope in InvalidProcessingGroupType because that variant is what makes ProcessingError huge.

#skip-changelog