Add Container.OnDisposing event for ordered teardown

[svermeulen]

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Description

Adds a public event Action<Container> OnDisposing to Container. It fires at the very top of Container.Dispose() — before children are disposed and before the auto-dispose pass over registered IDisposable instances.

Today, Container.Dispose() runs in a fixed order: child containers in reverse, then resolved/registered IDisposable instances popped in reverse-construction order. There is no extension point for application code to coordinate its own shutdown sequence before this happens.

A single event on Container, fired as the first line of Dispose(). Subscribers can run their own ordered shutdown logic against still-live dependencies, then let the standard teardown proceed.

Also, when combined with #146, allows for complete customization of dispose order

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

Ran via Unity Test Runner. All tests passed (though GarbageCollectionTests were skipped since it was run in debug)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have checked my code and corrected any misspellings
• I have checked that Reflex.GettingStarted still runs nicely