Clean transient entities and preserve state-included metadata

[Copilot]

CleanEntityStorageAsync ignored transient entities and entity queries reported IncludeState=false when state was missing, leaving empty/transient entities behind.

• Entity cleaning: After server-side clean, traverse transient entities and purge those with no state/locks, ensuring empty entities are removed.
• Metadata correctness: Preserve IncludesState=true even when state payload is empty/null; avoid null dereference and use consistent empty-state detection.
• Coverage: Added gRPC client tests for transient purge and state-included metadata scenarios.

var result = await client.Entities.CleanEntityStorageAsync(
    new CleanEntityStorageRequest { RemoveEmptyEntities = true, ReleaseOrphanedLocks = true });
// Transient empty entities are now removed; state-included metadata reports IncludesState=true even when state is null.

Original prompt

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This section details on the original issue you should resolve

<issue_title>CleanEntityStorageAsync() does not clean entities (likely 'transient' ones)</issue_title>
<issue_description>Hi! I am migrating some code from from in-process durable framework to isolated model, and I got stuck with handling entities.
I was looking for an option to completely delete entities. Reading docs, I learned that I can use DurableTaskClient.CleanEntityStorageAsync() to clean storage. I manually delete state for entities I wish to query and then I call this method, which does nothing, reporting that it purged exactly 0 entities.
Now my entities in question appear to be 'transient' so that I need to pass new EntityQuery { IncludeTransient = true } to GetAllEntitiesAsync() in order to see them in the output. It feels like by deleting state I promoted entities to transient and now entity storage is not cleaned because of that. The scenario for debugging is roughly the following:

// Here I get two transient entities with no state
var entities = await client.Entities
  .GetAllEntitiesAsync(
      new EntityQuery
      {
          IncludeState = true,
          IncludeTransient = true
      }
  )
  .ToListAsync();

// And here I get 0 removed entities and 0 orphaned locks released
var result = await client.Entities.CleanEntityStorageAsync(
    new CleanEntityStorageRequest
    {
        RemoveEmptyEntities = true,
        ReleaseOrphanedLocks = true
    }
  );

// Here again I get two entities
var entitiesAfter = await client.Entities
  .GetAllEntitiesAsync(
      new EntityQuery
      {
          IncludeState = true,
          IncludeTransient = true
      }
  )
  .ToListAsync();

Perhaps I do not understand something about this flow?

Also, when querying for entities, even though I specify IncludeState = true, resulting metadata do not include state and have IncludesState set to false. Is this how it is supposed to work?

I debug this code locally, backing durable framework by Azurite storage, so I can access and inspect my entities, and check if they are removed or not, or if they have associated state.</issue_description>

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

• Fixes CleanEntityStorageAsync() does not clean entities (likely 'transient' ones) #324

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