chore(core): verify sync envelope if signed

[jrriehl]

Proposed Changes

A previous update left open the possibility that an envelope sent as a sync response with a signature would not be verified. With this change, we ensure that any envelope that contains a signature gets verified.

Types of changes

What type of change does this pull request make (put an x in the boxes that apply)?

• Bug fix (non-breaking change that fixes an issue).
• New feature added (non-breaking change that adds functionality).
• Breaking change (fix or feature that would cause existing functionality to stop working as expected).
• Documentation update.
• Something else (e.g., tests, scripts, example, deployment, infrastructure).

Checklist

Put an x in the boxes that apply:

• I have read the CONTRIBUTING guide
• Checks and tests pass locally