Add support for complex type discriminator.

[AndriySvyryd]

Breaking change: IDiscriminatorPropertySetConvention.ProcessDiscriminatorPropertySet signature changed

Part of #31376

PR Overview

This PR adds support for complex type discriminators and updates related APIs and conventions by breaking the signature of IDiscriminatorPropertySetConvention.ProcessDiscriminatorPropertySet. Key changes include the introduction of new builders/interfaces for complex type discriminators, modifications to model validation and snapshot generation, and adjustments in convention dispatcher implementations to support the new discriminator configuration on both entity and complex types.

Reviewed Changes

File	Description
src/EFCore/Metadata/Builders/IConventionComplexTypeDiscriminatorBuilder.cs	New interface to provide a simple API for configuring complex type discriminators.
src/EFCore/Metadata/Builders/ComplexTypeDiscriminatorBuilder`.cs	New implementation for configuring discriminator values for complex types (note the extraneous backtick in the filename).
src/EFCore/Infrastructure/ModelValidator.cs	Updates to validate discriminator values for both entity and complex types.
src/EFCore.Design/Migrations/Design/CSharpSnapshotGenerator.cs	Adjustments to generate fluent API calls for discriminator configuration in the snapshot.
src/EFCore.Cosmos/Metadata/Conventions/CosmosJsonIdConvention.cs	Changes refactoring chained calls for discriminator configuration on Cosmos DB.
src/EFCore/Metadata/Conventions/DiscriminatorConvention.cs	Updated signature handling in discriminator processing to support IConventionTypeBaseBuilder.
Other files (ConventionDispatcher, IConventionTypeBaseBuilder, etc.)	Various updates across the conventions infrastructure to support the breaking change.

Copilot reviewed 67 out of 67 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (3)

src/EFCore/Metadata/Builders/ComplexTypeDiscriminatorBuilder`.cs:1

• The file name 'ComplexTypeDiscriminatorBuilder`.cs' contains an extraneous backtick; consider renaming it to 'ComplexTypeDiscriminatorBuilder.cs' for consistency.

+// Licensed to the .NET Foundation under one or more agreements.

src/EFCore.Relational/Infrastructure/RelationalModelValidator.cs:1960

• [nitpick] Consider adding tests that explicitly cover the validation of discriminator values on nested complex types to ensure comprehensive validation.

+                    foreach (var complexProperty in derivedType.GetDeclaredComplexProperties())

src/EFCore/Metadata/Conventions/Internal/ConventionDispatcher.DelayedConventionScope.cs:83

• [nitpick] Ensure that the use of 'structuralTypeBuilder' for the parameter in OnDiscriminatorPropertySet is consistently applied across all dispatcher implementations for clarity.

public override string? OnDiscriminatorPropertySet(IConventionTypeBaseBuilder structuralTypeBuilder, string? name)