chore: use safer slice splitting

[AaronFeickert]

Description

Ensures that all slice splitting is properly checked to avoid panics.

Motivation and Context

There are a few remaining uses of slice::split_at that are used to split slices. Because this can panic on a bad index, this PR moves each use to slice::split_at_checked, which cannot panic.

How Has This Been Tested?

Existing tests pass.

What process can a PR reviewer use to test or verify this change?

Ensure that each split is still being done at the same index, and that the error handling is done properly.

[AaronFeickert]

Note that this does not include the uses already addressed by #6430.

[github-actions]

Test Results (CI)

    3 files    116 suites   41m 43s ⏱️
1 301 tests 1 300 ✅ 0 💤 1 ❌
3 785 runs  3 784 ✅ 0 💤 1 ❌

For more details on these failures, see this check.

Results for commit 3b4dcdd.

♻️ This comment has been updated with latest results.

[github-actions]

Test Results (Integration tests)

 2 files  11 suites   24m 19s ⏱️
35 tests 33 ✅ 0 💤 2 ❌
37 runs  35 ✅ 0 💤 2 ❌

For more details on these failures, see this check.

Results for commit 3b4dcdd.

[SWvheerden]

I looked at these and found they are not a danger at this moment as they don't take in extra outside input or they check it before hand,
But the work here is done and that stuff might change in future.

[AaronFeickert]

I also didn't identify any particular problems with the existing implementation. As noted, this was to reduce risk going forward in the event of changes.