.NET: Consolidate InMemoryAgentSessionStore in Microsoft.Agents.AI

### Description

Track Wesley's follow-up suggestion from PR #7991: https://github.com/microsoft/agent-framework/pull/7991#discussion_r4007878901.

Promote a reusable `InMemoryAgentSessionStore` to the `Microsoft.Agents.AI` package, alongside `ChatClientAgent` and `DelegatingAgentSessionStore`, and consolidate the existing implementations in Hosting and Foundry.Hosting. This is intentionally out of scope for #7991.

Both implementations store serialized sessions in concurrent dictionaries and deserialize independent session instances on lookup. Their agent identity behavior differs: Hosting uses `agent.Id`; Foundry.Hosting uses a resolved hosting identity, with an instance-ID fallback for unnamed agents. Define how that hosting-specific behavior is preserved without adding Foundry dependencies to the core package.

Expected outcome:
- One reusable core implementation, with hosting packages using it or a thin adapter where identity behavior requires one.
- Preserve partition isolation, independent restored sessions, and experimental API annotations.
- Update registrations, public API declarations, documentation, and tests.

### Code Sample

Proposed usage after promotion:

```csharp
using Microsoft.Agents.AI;

AgentSessionStore store = new InMemoryAgentSessionStore();
```

### Language/SDK

.NET

Start by comparing the existing InMemoryAgentSessionStore implementations in Hosting and Foundry.Hosting, along with their registrations, public API declarations, documentation, and tests. Define how hosting-specific agent identity can remain available without Foundry dependencies in Microsoft.Agents.AI. Done means one reusable core implementation or a thin adapter, with partition isolation, independent restored sessions, and experimental API annotations preserved.

Written by the indexing model from the issue text.