.NET: [Feature]: Allow customizing partition key strategy in `CosmosChatHistoryProvider`

@westey-m is already working on this.

Since May 5, 2026.

### Description

### Problem

`CosmosChatHistoryProvider` couples two concerns: the partition key shape and which fields land on the document. Setting `State.TenantId` and `State.UserId` is what triggers hierarchical partitioning *and* what causes those values to be persisted on documents — they can't be decoupled. So there's no way to use flat `/conversationId` partitioning while still storing tenant/user metadata for filtering, RBAC scoping, or change-feed routing.

### Motivation

Multi-tenant chatbots that support unauthenticated → authenticated user transitions need both: flat PK for continuity across the auth boundary, *and* tenant/user fields on documents for tenant isolation and admin queries. Currently you have to give up one.

### Proposal

Two separable, backward-compatible changes:

1. **Always persist `TenantId`/`UserId`/`SessionId` on documents when set on `State`**, regardless of partition key shape.
2. **Add an optional `Func` constructor parameter** to override `BuildPartitionKey`. Defaults to current behavior when null.

Smaller interim alternative: remove `sealed` and mark `BuildPartitionKey`, `UseHierarchicalPartitioning`, and `CreateMessageDocument` as `protected virtual`.

### Alternatives considered

- Storing tenant/user inside `ChatMessage` metadata — values get buried in the serialized `message` blob; no efficient filtering.
- Writing a fully custom `ChatHistoryProvider` — viable but requires reimplementing transactional batching, request-too-large splitting, TTL, and batch validation.

Source: `dotnet/src/Microsoft.Agents.AI.CosmosNoSql/CosmosChatHistoryProvider.cs`

### Language/SDK

.NET