Fix UmbracoMapper InvalidOperationException due to concurrent modification of inner dictionary

[albinj]

Prerequisites

• I have added steps to test this contribution in the description below

Description

Fixes issue #15830 by changing the inner Dictionary of _ctors to a ConcurrentDictionary.

This concurrency issue was encountered when mapping one type of object to multiple other objects (multiple mapping definitions) and performing multiple concurrent requests when Umbraco boots to a url with a view that did all the mappings at once.

Error: Umbraco.Cms.Web.Common.PublishedModels.FrontPage => Site.MyModel
System.InvalidOperationException: Operations that change non-concurrent collections must have exclusive access. A concurrent update was performed on this collection and corrupted its state. The collection's state is no longer correct.
at System.Collections.Generic.Dictionary`2.FindValue(TKey key)
at System.Collections.Generic.Dictionary`2.TryGetValue(TKey key, TValue& value)
at Umbraco.Cms.Core.Mapping.UmbracoMapper.GetCtor(Type sourceType, Type targetType)
at Umbraco.Cms.Core.Mapping.UmbracoMapper.Map[TTarget](Object source, Type sourceType, MapperContext context)
at Umbraco.Cms.Core.Mapping.UmbracoMapper.Map[TTarget](Object source, MapperContext context)
at Umbraco.Cms.Core.Mapping.MapperContext.Map[TTarget](Object source)

The stack trace points to the UmbracoMapper.GetCtor method as the origin of the exception, indicating that the issue occurs during the retrieval of constructor mappings in concurrent scenarios.

Steps to test

• Implement an UmbracoMapper mapping, for example, mapping from Umbraco.Cms.Web.Common.PublishedModels.FrontPage to Site.MyModel.
• Use the mapper in a razor view
• Simultaneously send multiple requests during Umbraco startup to the view rendering the mapped models using tools like curl to simulate concurrent access.
• Monitor the application for any instances of InvalidOperationException indicating corrupted state due to concurrent modifications.
• If it does not work, restart Umbraco and try again.
• This PR fixes the InvalidOperationException from UmbracoMapper

[github-actions]

Hi there @albinj, thank you for this contribution! 👍

While we wait for one of the Core Collaborators team to have a look at your work, we wanted to let you know about that we have a checklist for some of the things we will consider during review:

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[georgebid]

Hey @albinj, thanks a lot for your PR to fix #15830! Someone on the core collaborators team will review this soon 😄

[nul800sebastiaan]

Thanks @albinj - looks good to me, thanks for the assist! 👍