Fix issue with generated code when using json_serializer

[frazeradam]

The Bug

When using merge_outputs: true with json_serializer: json_serializable, the generated file incorrectly includes:

part 'ffs_api.freezed.dart';
part 'ffs_api.g.dart';

It should only include:

part 'ffs_api.g.dart';

Why it's wrong

json_serializable only needs .g.dart files (for @JsonSerializable classes)
freezed needs both .freezed.dart and .g.dart files (for @Freezed classes)
The code generator correctly uses @JsonSerializable annotations (not @Freezed), but the part directive for freezed is still being added for both serializers.

Cause

The merged file template unconditionally adds part 'filename.freezed.dart' regardless of which json_serializer is configured. It should only add that line when the configuration is json_serializer: freezed.

Evidence:

With merge_outputs: false, each file correctly only has part 'filename.g.dart'
With merge_outputs: true, the freezed part directive appears even with json_serializer: json_serializable

Tests

A test has been added to make sure the changes work as desired, and other tests continue to pass.

---

This PR was primarily done by @TDLehman, I just did the review and got the description cleaned up and entered.

[frazeradam]

This is a pretty simple fix, so I didn't bother creating a separate issue for it to resolve, but if you'd like us to do so, we can.

[frazeradam]

@Carapacik anything you need from us to get this PR reviewed?

[Carapacik]

Thanks