Fix casting floats to Decimal64

[AdamGS]

Which issue does this PR close?

Closes #8362

Rationale for this change

Fixes casting codepaths that currently fail.

What changes are included in this PR?

Are these changes tested?

yes

If tests are not included in your PR, please explain why (for example, are they covered by existing tests)?

Are there any user-facing changes?

Code should now behave as expected

[alamb]

Thank you for this fix @AdamGS and for the review @mbrobbel

[alamb]

FWIW the new test fails like this without the changes to the code

Failed to cast to Decimal64(9, 3) with: Cast error: Cannot cast to Decimal64(9, 3). Overflowing on 1.1
thread 'cast::tests::test_cast_floating_to_decimals' panicked at arrow-cast/src/cast/mod.rs:3125:13:
Failed to cast to Decimal64(9, 3) with: Cast error: Cannot cast to Decimal64(9, 3). Overflowing on 1.1
stack backtrace: