Add IsProjected support to selector-based projections

[glen-84]

Summary

• IsProjected() and [IsProjected] are now honored when projections are built from the selection set through QueryContext<T> or a DataLoader Select(selection) call, matching the projection middleware: always-projected leaf fields backed by a property are fetched even when not selected, and never-projected fields are skipped even when selected (resolving to their default value).
• Breaking: the public HotChocolate.Data.Projections.ProjectionFeature record is removed. Its state is now carried by internal CoreFieldFlags (AlwaysProjected, NotProjected, HasProjectionMiddleware), read through a new internal helper on IOutputFieldDefinition. HasProjectionMiddleware gets its own flag instead of reusing UsesProjections, which [SkipDataLoaderCache] also sets.
• The 15-to-16 migration guide documents the removal, noted as landing in 16.6.2.

Test plan

• New IsProjectedSelectorTests (Sqlite with SQL capture) pin the exact SQL: always-projected fetched when unselected, never-projected omitted when selected, __typename-only selection, and the DataLoader Select(selection) path.
• Regression: Data.EntityFramework.Tests, Data.Tests, Data.Projections.Tests, the legacy IsProjected suites (SqlServer, MongoDb, Raven), and the PostgreSQL AsSelector suites all pass.

Closes #8313

[Copilot]

Pull request overview

This PR aligns selector-based projections (e.g., QueryContext<T> and DataLoader Select(selection)) with projection middleware behavior by honoring IsProjected() / [IsProjected] for both always-projected and never-projected fields, and it replaces the removed public ProjectionFeature record with internal CoreFieldFlags-based state.

Changes:

• Honor AlwaysProjected and NotProjected when building projections from selection sets (including __typename-only selections) and when using DataLoader selector paths.
• Remove HotChocolate.Data.Projections.ProjectionFeature and move its state to internal CoreFieldFlags (AlwaysProjected, NotProjected, HasProjectionMiddleware) plus an internal helper on IOutputFieldDefinition.
• Add EF Core Sqlite tests that snapshot the generated SQL to verify always/never projection behavior for selector-based projections.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
website/content/docs/hotchocolate/migrating/migrate-from-15-to-16.md	Documents the breaking removal of ProjectionFeature and directs users to IsProjected() / [IsProjected].
src/HotChocolate/Data/test/Data.EntityFramework.Tests/IsProjectedSelectorTests.cs	Adds SQL-snapshot integration tests for selector-based IsProjected behavior (QueryContext + DataLoader Select(selection)).
src/HotChocolate/Data/src/Data/Projections/ProjectionTypeInterceptor.cs	Switches always-projected field detection to internal core flags.
src/HotChocolate/Data/src/Data/Projections/ProjectionFeature.cs	Removes the public ProjectionFeature record (breaking API change).
src/HotChocolate/Data/src/Data/Projections/Extensions/ProjectionObjectFieldDescriptorExtensions.cs	Writes projection behavior into CoreFieldFlags and sets HasProjectionMiddleware when compiling projection middleware.
src/HotChocolate/Data/src/Data/Projections/Extensions/OutputFieldExtensions.cs	Reads projection behavior (NotProjected, AlwaysProjected, HasProjectionMiddleware) from core flags instead of Features.
src/HotChocolate/Core/src/Types/Types/Extensions/OutputFieldDefinitionExtensions.cs	Adds internal helper to read CoreFieldFlags from IOutputFieldDefinition implementations.
src/HotChocolate/Core/src/Types/Types/Descriptors/Configurations/CoreFieldFlags.cs	Introduces new internal flags used to represent projection state.
src/HotChocolate/Core/src/Execution.Projections/SelectionExpressionBuilder.cs	Skips NotProjected fields and injects AlwaysProjected leaf-property fields into selector-based projections.

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[github-actions]

Patch coverage

100.0% of changed lines covered (34/34)

File	Covered	Changed	Patch %
…/Core/src/Execution.Projections/SelectionExpressionBuilder.cs	17	17	100.0% 🟢
…/Types/Types/Extensions/OutputFieldDefinitionExtensions.cs	1	1	100.0% 🟢
…/src/Data/Projections/Extensions/OutputFieldExtensions.cs	3	3	100.0% 🟢
…/Extensions/ProjectionObjectFieldDescriptorExtensions.cs	11	11	100.0% 🟢
…/Data/src/Data/Projections/ProjectionTypeInterceptor.cs	2	2	100.0% 🟢

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Project coverage: 57.5% (283887/493502 lines)