.NET: [BREAKING] Decouple Cosmos chat metadata from partition-key strategy - #9214

Motivation & Context

CosmosChatHistoryProvider ties tenant/user metadata persistence to hierarchical partitioning. Flat conversation-ID partitioning should retain that metadata and preserve conversation continuity across authentication transitions.

Description & Review Guide

• What are the major changes?
  • Partition strategy: Add optional PartitionKeyFactory, used for writes, reads, counts, and deletes.
  • Metadata: Persist tenant/user IDs whenever provided; always persist session ID as conversation ID.
  • Configuration: Move optional constructor arguments into CosmosChatHistoryProviderOptions.
• What is the impact of these changes? Default partitioning remains unchanged. Callers supplying optional constructor arguments must migrate to options.
• What do you want reviewers to focus on? Strategy consistency across operations, metadata independence, and preservation of filtering and client-ownership behavior.

Flat partitioning with tenant/user metadata:

new CosmosChatHistoryProvider(
    cosmosClient, databaseId, containerId,
    _ => new CosmosChatHistoryProvider.State(conversationId, tenantId, userId),
    new CosmosChatHistoryProviderOptions
    {
        PartitionKeyFactory = state => new PartitionKey(state.ConversationId)
    });

Related Issue

Maintainer-approved direction, including the options-class refactor.

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Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR links to an agreed issue with no competing open PR, or the Related Issue section documents a trivial-change or repository-automation exception.
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

• Fixes .NET: [Feature]: Allow customizing partition key strategy in CosmosChatHistoryProvider #5601