[Joey0v0]

Fixes #4777

When an API type embeds another struct, goctl api dart fails with
cannot get property name of "" because the generator treats the
anonymous embedded member as a named field.

This change expands embedded fields while preserving their tags and
converting their Dart types. It supports nested and pointer embeddings
in both data templates, and handles embedded path parameters.

Regression tests cover embedded structs, custom JSON tags, field
conflicts, and both API parsers.

Validation:

• go test ./api/dartgen -count=1 passed.
• go vet ./api/dartgen passed.
• The API example from goctl api dart generates http api error #4777 generates successfully.
• Full goctl tests fail in api/format, docker, util/ctx, and util/pathx
  on Windows. These failures were also reproduced with unmodified source.
• Dart compilation was not checked because the Dart SDK is unavailable.