Closes #27974: Optional entity splitting fragments

[anasik]

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        Fixes #bugnumber

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[Copilot]

Pull request overview

This PR adds support for optional entity-splitting mapping fragments in EF Core Relational: a split table row can be considered optional (absent unless at least one non-key value exists), affecting query join shape, validation, update pipeline behavior, runtime model metadata, scaffolding/snapshot generation, and migrations diagnostics.

Changes:

• Introduces fragment optionality metadata (IsOptional / IsSplitFragmentOptional) and flows it through runtime model building and code generation.
• Updates relational query translation to use LEFT JOIN and nullable projections for optional fragments.
• Adjusts update batching and migrations/model validation to account for optional fragments, with added tests and diagnostics (new warning + event payload).

Reviewed changes

Copilot reviewed 40 out of 41 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
test/EFCore.SqlServer.FunctionalTests/Update/SqlServerUpdateSqlGeneratorTest.cs	Adds new usings (currently appear unused).
test/EFCore.SqlServer.FunctionalTests/Scaffolding/Baselines/No_NativeAOT/OwnedTypeEntityType.cs	Updates scaffold baseline for new runtime fragment ctor parameter.
test/EFCore.SqlServer.FunctionalTests/Scaffolding/Baselines/BigModel/OwnedTypeEntityType.cs	Updates scaffold baseline for new runtime fragment ctor parameter.
test/EFCore.Sqlite.FunctionalTests/Scaffolding/Baselines/No_NativeAOT/OwnedTypeEntityType.cs	Updates scaffold baseline for new runtime fragment ctor parameter.
test/EFCore.Sqlite.FunctionalTests/Scaffolding/Baselines/BigModel/OwnedTypeEntityType.cs	Updates scaffold baseline for new runtime fragment ctor parameter.
test/EFCore.Sqlite.FunctionalTests/Query/OptionalEntitySplittingQuerySqliteTest.cs	Adds query coverage for missing vs optional fragment rows and join shape.
test/EFCore.Sqlite.FunctionalTests/OptionalEntitySplittingSqliteTest.cs	Adds SaveChanges coverage for optional fragment insert/update/delete scenarios.
test/EFCore.Relational.Tests/Migrations/Internal/MigrationsModelDifferTestBase.cs	Updates MigrationsModelDiffer construction for new logger dependency.
test/EFCore.Relational.Tests/Migrations/Internal/MigrationsModelDifferTest.cs	Adds test for optionality-change warning during migration diffing.
test/EFCore.Relational.Tests/Infrastructure/RelationalModelValidatorTest.cs	Adds validation tests for optional fragments (nullable enforcement, mixed fragments).
test/EFCore.Relational.Specification.Tests/ModelBuilding/RelationalModelBuilderTest.cs	Adds model-building tests and test builders for IsOptional() on split tables.
test/EFCore.Design.Tests/Migrations/Design/MigrationScaffolderTest.cs	Updates MigrationsModelDiffer construction for new logger dependency.
src/EFCore.Relational/Update/ModificationCommand.cs	Adds optional-fragment helpers and changes Modified→Added decision logic.
src/EFCore.Relational/Update/Internal/CommandBatchPreparer.cs	Skips optional-fragment insert/delete commands when fragment row is effectively absent.
src/EFCore.Relational/Query/RelationalQueryableMethodTranslatingExpressionVisitor.CreateSelect.cs	Uses LEFT JOIN and nullable projections for optional fragments.
src/EFCore.Relational/Properties/RelationalStrings.resx	Adds new validation error strings and a migrations warning message.
src/EFCore.Relational/Properties/RelationalStrings.Designer.cs	Adds strongly-typed accessors and logging definition wiring for new resources.
src/EFCore.Relational/Migrations/Internal/MigrationsModelDiffer.cs	Injects migrations logger and emits optionality-change warning.
src/EFCore.Relational/Metadata/RuntimeEntityTypeMappingFragment.cs	Adds optionality to runtime mapping fragments and exposes it.
src/EFCore.Relational/Metadata/RelationalAnnotationNames.cs	Adds annotation name for fragment optionality.
src/EFCore.Relational/Metadata/ITableMappingBase.cs	Adds IsSplitFragmentOptional to table mapping metadata (debug string support).
src/EFCore.Relational/Metadata/IReadOnlyEntityTypeMappingFragment.cs	Adds IsOptional to fragment metadata (debug string support).
src/EFCore.Relational/Metadata/Internal/TableMappingBase.cs	Implements IsSplitFragmentOptional on internal mapping base.
src/EFCore.Relational/Metadata/Internal/RelationalModel.cs	Propagates fragment optionality into relational table/view mappings.
src/EFCore.Relational/Metadata/Internal/InternalEntityTypeMappingFragmentBuilder.cs	Adds internal builder APIs to set/check fragment optionality.
src/EFCore.Relational/Metadata/Internal/EntityTypeMappingFragment.cs	Stores optionality + configuration source and merges it during model building.
src/EFCore.Relational/Metadata/IMutableEntityTypeMappingFragment.cs	Adds mutable IsOptional API surface.
src/EFCore.Relational/Metadata/IConventionEntityTypeMappingFragment.cs	Adds convention API surface for fragment optionality.
src/EFCore.Relational/Metadata/Conventions/RelationalRuntimeModelConvention.cs	Emits runtime fragments with optionality propagated.
src/EFCore.Relational/Metadata/Builders/SplitTableBuilder`.cs	Adds public generic IsOptional() builder API.
src/EFCore.Relational/Metadata/Builders/SplitTableBuilder.cs	Adds non-generic IsOptional() builder API.
src/EFCore.Relational/Metadata/Builders/OwnedNavigationSplitTableBuilder``.cs	Adds public generic IsOptional() builder API for owned navigations.
src/EFCore.Relational/Metadata/Builders/OwnedNavigationSplitTableBuilder.cs	Adds non-generic IsOptional() builder API for owned navigations.
src/EFCore.Relational/Infrastructure/RelationalModelValidator.cs	Validates optional fragment constraints (nullable properties, no shared-table splitting).
src/EFCore.Relational/EFCore.Relational.baseline.json	Updates API baseline for new public APIs and signature changes.
src/EFCore.Relational/Diagnostics/RelationalLoggingDefinitions.cs	Adds logging definition slot for new migrations warning.
src/EFCore.Relational/Diagnostics/RelationalLoggerExtensions.cs	Adds logger extension emitting warning + DiagnosticSource payload.
src/EFCore.Relational/Diagnostics/RelationalEventId.cs	Adds new migrations event id for optionality-change warning.
src/EFCore.Relational/Diagnostics/EntityTypeMappingFragmentEventData.cs	Adds new DiagnosticSource payload type for fragment-related events.
src/EFCore.Relational/Design/Internal/RelationalCSharpRuntimeAnnotationCodeGenerator.cs	Emits IsSplitFragmentOptional and runtime fragment optionality into generated runtime model.
src/EFCore.Design/Migrations/Design/CSharpSnapshotGenerator.cs	Emits t.IsOptional() in snapshots for optional split fragments.

Files not reviewed (1)

• src/EFCore.Relational/Properties/RelationalStrings.Designer.cs: Generated file