Document DeleteObjectReference to prevent memory leaks in Orleans observers

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Calling CreateObjectReference without a matching DeleteObjectReference leaves a reference in InvokableObjectManager.localObjects, causing a memory leak that can eventually crash the client process. The observers article never mentioned this requirement.

Changes

• Intro paragraph: Added that DeleteObjectReference must be called when the observer is no longer needed, cross-referenced via <xref:Orleans.IGrainFactory.DeleteObjectReference*>.
• New cleanup example: Added code showing proper teardown after the existing subscription example:

  // Unsubscribe the observer when it's no longer needed.
  await friend.UnSubscribe(obj);
  
  // Delete the object reference to free resources and avoid a memory leak.
  _grainFactory.DeleteObjectReference<IChat>(obj);

• [!IMPORTANT] callout: Warns that omitting DeleteObjectReference leaks a reference in the client's internal object manager and can eventually crash the process.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Mention DeleteObjectReference when talking about Orleans Observers</issue_title>
<issue_description>### Describe the issue or suggestion

https://github.com/dotnet/docs/blob/main/docs/orleans/grains/observers.md?target=https://github.com

The observers page teaches calling CreateObjectReference to enable an Orleans client to register an IGrainObserver with the library. It does not mention that this call needs to be paired with DeleteObjectReference.

If the latter is not called when done—at least for the OutsideRuntimeClient—a small reference remains in the InvokableObjectManager.localObjects dictionary, effectively becoming a memory leak, that will kill the (client) process eventually.

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Associated WorkItem - 558079</issue_description>

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

• Fixes Mention DeleteObjectReference when talking about Orleans Observers #43439

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Internal previews

📄 File	🔗 Preview link
docs/orleans/grains/observers.md	Observers

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Pull request overview

This PR updates the Orleans observers documentation to explicitly call out that CreateObjectReference must be paired with DeleteObjectReference to avoid leaking registrations in the client and potentially crashing long-running client processes.

Changes:

• Adds DeleteObjectReference guidance to the introductory explanation of client observers.
• Adds a cleanup snippet showing unsubscribe + object reference deletion.
• Adds an [!IMPORTANT] warning emphasizing the memory-leak/crash risk when DeleteObjectReference is omitted.

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