fix(backend): Inboxのエラーをthrowせずreturnしている問題を修正

[kakkokari-gtyih]

What

Fixes a type confusion bug where exceptions are incorrectly returned instead of thrown. The calling code will attempt to process the Error as an Activity, causing it to fail with cryptic and confusing errors.

Why

Fix https://activitypub.software/TransFem-org/Sharkey/-/issues/796 エラーがエラーとしてログに残るようになる

Additional info (optional)

Checklist

• [x] Read the contribution guide
• [ ] Test working in a local environment
• [ ] (If needed) Add story of storybook
• [x] (If needed) Update CHANGELOG.md
• [ ] (If possible) Add tests

[github-actions[bot]]

このPRによるapi.jsonの差分 差分はありません。 Get diff files from Workflow Page

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 11.11111% with 8 lines in your changes missing coverage. Please review.

Project coverage is 39.86%. Comparing base (c1f19fa) to head (d7e7b9a). Report is 6 commits behind head on develop.

Files with missing lines	Patch %	Lines
...ges/backend/src/core/activitypub/ApInboxService.ts	11.11%	8 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #15022 +/- ## =========================================== - Coverage 39.98% 39.86% -0.12% =========================================== Files 1563 1563 Lines 197737 197741 +4 Branches 3644 3633 -11 =========================================== - Hits 79056 78835 -221 - Misses 118077 118299 +222 - Partials 604 607 +3 ```

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[syuilo]

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