.NET: Make ChatMessageStore and AIContextProvider context props settable

[westey-m]

Motivation and Context

#3194

Description

• Make properties on all context parameter classes for ChatMessageStore and AIContextProvider settable, so that a filter clause can be written without needing to clone the entire context.

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

This PR makes properties on context parameter classes settable to enable filter clauses without requiring full context cloning. The change supports the existing ChatMessageStoreMessageFilter pattern where filter functions need to modify context properties.

Changes:

• Made RequestMessages property settable on InvokingContext classes in both ChatMessageStore and AIContextProvider
• Made RequestMessages and ChatMessageStoreMessages properties settable on ChatMessageStore.InvokedContext
• Made RequestMessages and AIContextProviderMessages properties settable on AIContextProvider.InvokedContext

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 11 comments.

File	Description
dotnet/src/Microsoft.Agents.AI.Abstractions/ChatMessageStore.cs	Changed 3 properties from get-only to get/set to enable filter modifications
dotnet/src/Microsoft.Agents.AI.Abstractions/AIContextProvider.cs	Changed 3 properties from get-only to get/set to enable filter modifications

[Copilot]

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