chore: harden invariants around migrations

[codecov-commenter]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (d09091e) 78.52% compared to head (d64e335) 78.35%.

Files	Patch %	Lines
util/fibers/listener_interface.cc	56.25%	7 Missing :warning:
util/fibers/proactor_base.h	33.33%	2 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #177 +/- ## ========================================== - Coverage 78.52% 78.35% -0.17% ========================================== Files 98 98 Lines 7292 7305 +13 ========================================== - Hits 5726 5724 -2 - Misses 1566 1581 +15 ```

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