incompatible_disable_target_default_provider_fields: Disallow use of default providers for legacy "struct access"

https://bazel.build/extending/rules#migrating_from_legacy_providers describes that using "struct access" patterns is already a legacy approach, and the preferred "modern" approach is to use explicit providers. That is
```
ctx.atr.foo.default_runfiles
```

should instead become
```
ctx.attr.foo[DefaultInfo].default_runfiles
```

Addressing the default provider directly by strings - e.g., `ctx.attr.foo.data_runfiles` - has the potential for namespace clash. This has happened in the past. That is, `ctx.attr.foo.fieldname` can potentially return a provider rather than a data field. However, `ctx.attr.foo[fieldname]` will always return a provider.

Begin with the linked Bazel migration guidance and compare the legacy struct-access examples with explicit provider access. Identify the Bazel validation and tests governing these two access forms; done means legacy string-based default-provider access is rejected while bracketed explicit-provider access remains supported.

Written by the indexing model from the issue text.