revert: remove unknown from Fetch client body type union

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Latest commit: 375a19c320ea63bec018dcf35f6d91dbcb8cc0d9

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Name	Status	Preview	Comments	Updated (UTC)
hey-api-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 11, 2024 1:43pm

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 67.56%. Comparing base (cc50007) to head (375a19c). Report is 6 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1275 +/- ## ======================================= Coverage 67.56% 67.56% ======================================= Files 133 133 Lines 19841 19841 Branches 1784 1784 ======================================= Hits 13405 13405 Misses 6432 6432 Partials 4 4 ``` | [Flag](https://app.codecov.io/gh/hey-api/openapi-ts/pull/1275/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=hey-api) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/hey-api/openapi-ts/pull/1275/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=hey-api) | `67.56% <ø> (ø)` | | 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=hey-api#carryforward-flags-in-the-pull-request-comment) to find out more.

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