fix: fix insert ephemeral

[MrCroxx]

What's changed and what's your intention?

Please explain IN DETAIL what the changes are in this PR and why they are needed. :D

As title.

Remove the entry when the last ref is dropped (idempotently) if the entry is still ephemeral.

Checklist

• I have written the necessary rustdoc comments
• I have added the necessary unit tests and integration tests
• I have passed make all (or make fast instead if the old tests are not modified) in my local environment.

Related issues or PRs (optional)

#785

[codecov]

Codecov Report

Attention: Patch coverage is 95.58824% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
foyer-memory/src/raw.rs	95.58%	3 Missing ⚠️

Files with missing lines	Coverage Δ
foyer-memory/src/raw.rs	90.76% <95.58%> (+2.53%)	⬆️

... and 3 files with indirect coverage changes

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