Bring back the Derived Type Discriminator Convention for MongoDB

[Copilot]

Re-introduces the DerivedTypeDiscriminatorConvention that was accidentally dropped during repository migration. This convention enables MongoDB to correctly serialize and deserialize polymorphic types decorated with DerivedTypeAttribute using a _derivedTypeId GUID discriminator field.

Added

• DerivedTypeDiscriminatorConvention — implements IDiscriminatorConvention using _derivedTypeId as the BSON discriminator field; resolves derived types via IDerivedTypes.GetDerivedTypeFor and falls back to the nominal type when no discriminator is present
• Auto-registration in MongoDBDefaults.Initialize — iterates IDerivedTypes.TypesWithDerivatives and registers the convention per base type via BsonSerializer.RegisterDiscriminatorConvention; no-ops gracefully if IDerivedTypes is not configured

// Types decorated with [DerivedType] will be stored with a GUID discriminator
[DerivedType("b91c21cd-56db-4edc-805b-b1d1ff9aa772", typeof(BaseDocument))]
public class SpecificDocument : BaseDocument { ... }

// MongoDB document stored as:
// { "_derivedTypeId": "b91c21cd-56db-4edc-805b-b1d1ff9aa772", ... }

Original prompt

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This section details on the original issue you should resolve

<issue_title>Bring back the Derived Type serializer for MongoDB</issue_title>
<issue_description>We used to have a Derived Type serializer, but it got accidently removed during transitioning of repositories.
Bring this back from this fork:

https://github.com/einari/MongoDB/blob/main/Source/DerivedTypeDiscriminatorConvention.cs</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Bring back the Derived Type serializer for MongoDB #642

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