FloatingPointConverter fails on narrowing mantissa with explicitJBit

### Describe the bug

Tests left a hole on changing the mantissaWidth when the output is explicitJBit.

### To Reproduce

Run explicitToExplicit exhaustive with narrowing mantissa and a case will fail.

Need better exhaustive tests, including different exponent and mantissa widths, but also with explicitJBit and now DAZ/FTZ to be tested.

### Expected behavior

_No response_

### Actual behavior

_No response_

### Additional: Dart SDK info

_No response_

### Additional: pubspec.yaml

```yaml

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### Additional: Context

_No response_

Start with the explicitToExplicit exhaustive test mentioned in the issue and trace it to the FloatingPointConverter implementation. Reproduce the failure with a narrowing mantissa and explicitJBit, then broaden coverage across exponent and mantissa widths, explicitJBit, and DAZ/FTZ cases until the exhaustive tests pass.

Written by the indexing model from the issue text.