feat(selections): add nested selected paths

[pavelnikolov]

SelectedFieldNames now returns dot-delimited nested field paths (e.g. products, products.id, products.category, products.category.id). Intermediate container object/list paths are included so resolvers can check for both a branch (products.category) and its leaves (products.category.id). HasSelectedField and SortedSelectedFieldNames operate on these paths. This aligns behavior with typical resolver projection needs and fixes missing nested selections.

fix #679

[Copilot]

Pull Request Overview

This PR adds support for nested field paths in GraphQL field selection helpers. The SelectedFieldNames function now returns dot-delimited paths for nested selections (e.g., products.id, products.category.id) instead of just immediate child fields.

Key changes:

• Modified selection path collection to traverse nested structures recursively
• Updated API documentation to reflect the new nested path behavior
• Expanded test coverage to validate nested path functionality

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
selection.go	Updated API documentation and function behavior for nested paths
internal/selections/context.go	Refactored path collection logic to support recursive nested traversal
selection_test.go	Enhanced test schema and cases to validate nested path selection behavior
CHANGELOG.md	Added entry documenting the new nested field path feature

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}