Add ability to specify idbag (#415)

[jeff-hagen]

Closes #415

Adds the ability to specify a collection .AsIdBag(). Note that the NHibernate xml mapping model is a little odd:

• The column attribute on collection-id is always required even though a column element can technically be specified.
• The type attribute is required and must be a .net type (varchar will throw an exception)
• The idbag is identical to the bag mapping, with the 2 exceptions:
  • The idbag has a collection-id element
  • The idbag doesn't have one-to-many

[jeff-hagen]

@hazzik would you mind taking a look at my PR?

[Copilot]

Pull Request Overview

This PR adds the ability to specify an idbag collection type in FluentNHibernate mappings. Idbag is a type of collection mapping in NHibernate that combines characteristics of a bag with an identifier column for each row. The implementation includes mapping configuration, XML generation, and visitor pattern support.

• Adds Collection.IdBag enum value and associated mapping infrastructure
• Implements CollectionIdMapping class to configure the collection identifier
• Adds AsIdBag() methods to fluent mapping API with configurable identifier type and generation strategy

Reviewed Changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
Collection.cs	Adds IdBag enum value to collection types
CollectionMapping.cs	Adds support for CollectionId property and IdBag factory method
CollectionIdMapping.cs	New mapping class for collection identifier configuration
ToManyBase.cs	Adds fluent AsIdBag() methods for configuring idbag collections
XmlIdBagWriter.cs	New XML writer for generating idbag elements
XmlCollectionIdWriter.cs	New XML writer for collection-id elements
XmlCollectionWriter.cs	Updates collection writer to handle IdBag type
Visitor classes	Adds support for visiting CollectionIdMapping
GeneratorBuilder.cs	Refactors default generator logic into reusable method
CollectionIdPart.cs	New fluent configuration class for collection identifiers
Test files	Adds comprehensive tests for idbag functionality

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