Increase test coverage `keras_core/model`

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: -21.28% :warning:

Comparison is base (4b7ede4) 76.21% compared to head (db463a7) 54.94%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #862 +/- ## =========================================== - Coverage 76.21% 54.94% -21.28% =========================================== Files 329 329 Lines 31270 31270 Branches 6083 6083 =========================================== - Hits 23833 17181 -6652 - Misses 5844 12726 +6882 + Partials 1593 1363 -230 ``` | [Flag](https://app.codecov.io/gh/keras-team/keras-core/pull/862/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team) | Coverage Δ | | |---|---|---| | [keras_core](https://app.codecov.io/gh/keras-team/keras-core/pull/862/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team) | `54.94% <ø> (-21.18%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team#carryforward-flags-in-the-pull-request-comment) to find out more. [see 189 files with indirect coverage changes](https://app.codecov.io/gh/keras-team/keras-core/pull/862/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=keras-team)

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