FixedPoint and FixedPointValue have different default values for signed

### Describe the bug

FixedPoint defaults to a signed version whereas FixedPointValue defaults to an unsigned version.

They should be the same, but this is a breaking change as there could be reliance on the default.

### To Reproduce

_No response_

### Expected behavior

_No response_

### Actual behavior

_No response_

### Additional: Dart SDK info

_No response_

### Additional: pubspec.yaml

```yaml

```

### Additional: Context

_No response_

Locate the definitions of FixedPoint and FixedPointValue and inspect how each sets its default for signed. Compare any existing tests around these components, then make their defaults consistent and add or update a regression test that verifies the selected behavior.

Written by the indexing model from the issue text.