.NET: [BREAKING] Change *Provider StateKey to list of StateKeys

[westey-m]

Motivation and Context

It's not good to assume that AIContextProviders and ChatHistoryProviders will only use a single state key. This is especially relevant if one of these becomes a proxy for multiple internal providers, in which case each would have their own keys, and these would all need to be surfaced.

Description

• Change string StateKey to IReadOnlyList<string> StateKeys to allow multiple state keys per *provider.
• Update implementations and tests.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

Updates the .NET agent provider abstractions to support multiple state keys per provider (rather than assuming a single key), and adjusts agent validation plus tests/samples accordingly.

Changes:

• Replace StateKey with StateKeys : IReadOnlyList<string> on AIContextProvider and ChatHistoryProvider (and update concrete providers).
• Update ChatClientAgent validation logic to detect key clashes across all declared state keys.
• Update unit tests and samples to use StateKeys.

Reviewed changes

Copilot reviewed 21 out of 21 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.Abstractions/AIContextProvider.cs	Switch provider state identifier to multi-key StateKeys.
dotnet/src/Microsoft.Agents.AI.Abstractions/ChatHistoryProvider.cs	Switch provider state identifier to multi-key StateKeys.
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs	Validate uniqueness/clashes across all keys (including override provider validation).
dotnet/src/Microsoft.Agents.AI/TextSearchProvider.cs	Implement StateKeys for TextSearchProvider.
dotnet/src/Microsoft.Agents.AI/Memory/ChatHistoryMemoryProvider.cs	Implement StateKeys for ChatHistoryMemoryProvider.
dotnet/src/Microsoft.Agents.AI.Abstractions/InMemoryChatHistoryProvider.cs	Implement StateKeys for InMemoryChatHistoryProvider.
dotnet/src/Microsoft.Agents.AI.CosmosNoSql/CosmosChatHistoryProvider.cs	Implement StateKeys for CosmosChatHistoryProvider.
dotnet/src/Microsoft.Agents.AI.Mem0/Mem0Provider.cs	Implement StateKeys for Mem0Provider.
dotnet/src/Microsoft.Agents.AI.FoundryMemory/FoundryMemoryProvider.cs	Implement StateKeys for FoundryMemoryProvider.
dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowChatHistoryProvider.cs	Implement StateKeys for WorkflowChatHistoryProvider.
dotnet/tests/Microsoft.Agents.AI.UnitTests/Memory/ChatHistoryMemoryProviderTests.cs	Update assertions to StateKeys.
dotnet/tests/Microsoft.Agents.AI.UnitTests/Data/TextSearchProviderTests.cs	Update assertions to StateKeys.
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgentTests.cs	Update tests/messages for new key validation behavior.
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgent_BackgroundResponsesTests.cs	Update mocks to provide StateKeys.
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgent_ChatHistoryManagementTests.cs	Update mocks to provide StateKeys.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AIContextProviderDecorators/AIContextProviderChatClientTests.cs	Update test provider override to StateKeys.
dotnet/tests/Microsoft.Agents.AI.Mem0.UnitTests/Mem0ProviderTests.cs	Update assertions to StateKeys.
dotnet/tests/Microsoft.Agents.AI.CosmosNoSql.UnitTests/CosmosChatHistoryProviderTests.cs	Update assertions to StateKeys.
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/InMemoryChatHistoryProviderTests.cs	Update assertions to StateKeys.
dotnet/samples/01-get-started/04_memory/Program.cs	Update sample provider override to StateKeys.
dotnet/samples/02-agents/Agents/Agent_Step04_3rdPartyChatHistoryStorage/Program.cs	Update sample provider override to StateKeys.