fix(pkl-schema): emit hints for fields under SubResources without a FieldHint

[JeroenSoeters]

Summary

• The PKL hints flattener in internal/schema/pkl/schema/formae.pkl skipped any nested SubResource-typed field whose declaration did not carry its own @FieldHint. @FieldHint annotations on fields three or more levels deep — e.g. Volumes.EFSVolumeConfiguration.RootDirectory on AWS::ECS::TaskDefinition — never reached the compiled schema.
• Consequence: the recursive patch-time stripper had no hint path to walk, AWS-populated provider defaults stayed in stored state, and any createOnly ancestor triggered a spurious resource replacement on reapply. This is the root cause behind the lgtm-task spurious-replace investigation on fix/ecs-spurious-replace-investigation and the reason the plugin-side annotation added in formae-plugin-aws#47 appeared to have no effect end-to-end.
• Fix: descend into any SubResource-typed field regardless of whether the declaration has an @FieldHint, and fall back to the default key transformation when no FieldHint is present to supply outputField. Same change in both hints() and the recursive subHintsWithVisited().
• Regression test added under internal/schema/pkl/schema/tests/formae.pkl that models the ECS pattern (outer field annotated, intermediate SubResource field unannotated, deepest field annotated) and asserts the 3-level hint emits with its annotations intact. The full test-pkl, lint, and test-all suites pass.