.NET: Add Cosmos NoSQL vector memory sample

[Saurish (nos-redacted)]

Motivation & Context

The .NET AgentWithMemory samples demonstrate progressively richer memory providers, but they do not show how to use Azure Cosmos DB for NoSQL as a persistent vector store. This adds an Azure-first reference for semantic chat-history recall across independent agent sessions while reusing the existing ChatHistoryMemoryProvider and VectorStore abstractions.

Description & Review Guide

• What are the major changes?
  • Adds Step 08, which configures CosmosNoSqlVectorStore with a Microsoft Foundry embedding deployment and passes it to ChatHistoryMemoryProvider.
  • Uses a session-specific storage scope and a user-wide search scope, then runs two separate AgentSession instances to demonstrate cross-session recall.
  • Configures the Cosmos client for System.Text.Json, creates the configured database when absent, and lets the connector create the vector-enabled collection.
  • Registers the sample in the parent index, solution, and automated sample verifier.
  • Adds the CommunityToolkit Cosmos NoSQL connector and raises the central Microsoft.Azure.Cosmos version from 3.54.0 to 3.61.0, which is required by connector 1.0.0.
• What is the impact of these changes?
  • This is an additive sample and does not change Agent Framework public APIs.
  • Focused sample and verifier builds, format verification, solution registration, and the existing Cosmos unit-test slice pass. The verifier discovers the new sample and skips cleanly when its required endpoints are unset.
  • A live Foundry/Cosmos run was not performed because those service endpoints are not configured. The full solution build reached unrelated GitHub Copilot samples that could not download a CLI archive from public npm because of the managed network's TLS restrictions.
• What do you want reviewers to focus on?
  • The Cosmos client serializer configuration, the storage/search scope choices, and the central Cosmos SDK version bump required by the connector.

Related Issue

Fixes #7551

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 is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• 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.

[Copilot]

Pull request overview

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Adds a new .NET “AgentWithMemory” sample demonstrating persistent, cross-session chat history recall backed by Azure Cosmos DB for NoSQL vector search.

Changes:

• Introduces Step 08 sample using CosmosNoSqlVectorStore + ChatHistoryMemoryProvider to persist and recall chat history across AgentSessions.
• Registers the sample in the parent README, solution, and sample verifier.
• Updates central package versions to include the CommunityToolkit Cosmos NoSQL vector connector and bumps Microsoft.Azure.Cosmos.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
dotnet/samples/02-agents/AgentWithMemory/README.md	Adds Step 08 link to the sample index.
dotnet/samples/02-agents/AgentWithMemory/AgentWithMemory_Step08_MemoryUsingCosmosNoSql/README.md	Documents prerequisites, env vars, and behavior for the new sample.
dotnet/samples/02-agents/AgentWithMemory/AgentWithMemory_Step08_MemoryUsingCosmosNoSql/Program.cs	Implements the Cosmos NoSQL vector-store-backed memory sample and cross-session recall.
dotnet/samples/02-agents/AgentWithMemory/AgentWithMemory_Step08_MemoryUsingCosmosNoSql/AgentWithMemory_Step08_MemoryUsingCosmosNoSql.csproj	Adds the new sample project and dependencies.
dotnet/eng/verify-samples/AgentsSamples.cs	Registers the new sample with the verifier and required env vars.
dotnet/agent-framework-dotnet.slnx	Adds the sample project to the solution.
dotnet/Directory.Packages.props	Adds CommunityToolkit Cosmos NoSQL connector version and bumps Microsoft.Azure.Cosmos.

[Saurish (nos-redacted)]

Follow-up commit 862b907 addresses all four Copilot comments: embedding dimensions are configurable and validated, sample memory is isolated with a per-run user scope, and the unused direct Newtonsoft.Json reference is removed. The affected sample builds successfully with dotnet build --no-restore, editor diagnostics are clean, git diff --check passes, and all review threads are resolved. The PR is now ready for review; the contributor license check is currently queued. westey (@westey-m), could you review this AgentWithMemory sample when available?

[Saurish (nos-redacted)]

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