fix: update backoff methods return type to include string - #409

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Greptile Summary

This PR tightens return-type declarations for the two backoff-related methods. Job::backoff() gains string in its union type (both PHP and PHPDoc) to reflect that the stored payload value is a comma-separated string written by getJobBackoff; Queue::getJobBackoff() is narrowed from mixed to ?string, which is precise since it always returns null or a Collection::implode result.

• Job::backoff() return type updated from array|int|null to array|int|string|null, matching what is actually read back from the job payload.
• Queue::getJobBackoff() return type tightened from mixed to ?string; the method body only ever returns null or a comma-joined string, so this is strictly more accurate.

Confidence Score: 5/5

Two-line type-declaration update with no behavioural change; safe to merge.

Both changes are pure type-annotation corrections that bring declarations in line with what the methods already return. No logic is altered, and the narrowed types are verified against all call sites.

No files require special attention.

Important Files Changed

_{Reviews (1): Last reviewed commit: "fix: update return type for backoff meth..." | Re-trigger Greptile}