Fix race in Hierarchy.TryCreateLogger

[gdziadkiewicz]

This PR adds the regression (compared to log4net version 2) test case from #292 and fix in form of bringing back lock in Hierarchy.TryCreateLogger that is used to synchronize creation and registering of new Loggers. Prior to the change it was possible to get an unregistered logger that will result in dropped messages.

Resolves #292

[gdziadkiewicz]

Two of the new tests are on the slow list :

1. log4net.Tests.Hierarchy.LoggingConcurrencyTest.GetLogger_UnderHighConcurrency_ShouldNotLoseMessages (2.56 s)
2. log4net.Tests.Hierarchy.LoggingConcurrencyTest.GetLogger_WithDottedNamesUnderHighConcurrency_ShouldNotLoseMessages (2.13 s)

[Copilot]

Pull request overview

This pull request addresses a concurrency regression in Hierarchy.GetLogger(...) where concurrent logger creation could return an unregistered Logger, causing events to be dropped. It restores synchronization around logger creation/registration and adds regression tests based on the reproduction in #292.

Changes:

• Reintroduces a lock in Hierarchy.TryCreateLogger to serialize logger creation + registration, avoiding returning partially registered loggers.
• Introduces an internal IHierarchyNode marker so the logger map can store either Logger or ProvisionNode in a type-safe way.
• Adds concurrency and repository enumeration regression tests, including cases with dotted logger names and failure scenarios in the logger-creation event.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/log4net/Repository/Hierarchy/ProvisionNode.cs	Adds IHierarchyNode and implements it for ProvisionNode to support typed storage in the hierarchy map.
src/log4net/Repository/Hierarchy/Logger.cs	Implements IHierarchyNode on Logger to unify dictionary value types.
src/log4net/Repository/Hierarchy/Hierarchy.cs	Fixes the race by locking around logger creation/registration and updates hierarchy node handling to use IHierarchyNode.
src/log4net.Tests/Hierarchy/LoggingConcurrencyTest.cs	Adds regression tests to validate no message loss and correct blocking behavior during concurrent logger creation/registration.
src/log4net.Tests/Hierarchy/HierarchyTest.cs	Adds tests ensuring GetCurrentLoggers() behavior (empty, includes created, excludes root, excludes provision nodes).

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[FreeAndNil]

Thanks a lot.