.NET: Add serialization support for ContextualFunctionProvider

[Copilot]

Motivation and Context

Addresses review feedback from #2505 to add state persistence for ContextualFunctionProvider, enabling restoration of recent message context across sessions or in distributed scenarios.

Description

Implements serialization/deserialization following the TextSearchProvider pattern:

Core Changes:

• Added constructor overload accepting JsonElement serializedState with deserialization logic
• Implemented Serialize() method to persist recent messages queue as ChatMessage objects
• Created ContextualFunctionProviderState class and registered with AgentJsonUtilities.JsonContext
• Respects NumberOfRecentMessagesInContext limit during deserialization, truncating if changed

Usage:

// Serialize provider state
JsonElement state = provider.Serialize();

// Restore from serialized state
var restoredProvider = new ContextualFunctionProvider(
    vectorStore,
    vectorDimensions,
    functions,
    maxNumberOfFunctions,
    serializedState: state,
    options: options);

Testing:

• 5 new unit tests covering empty state, serialization, deserialization, round-trip, and limit truncation
• Updated test to demonstrate truncation behavior with limit of 2 messages (3 messages added, only 2 most recent kept)
• All 345 existing tests pass

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.

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[westey-m]

@copilot , change this to 2, so that we can see messages truncation in action.

[Copilot]

Changed to 2. The test now demonstrates truncation by adding 3 messages (M1, M2, M3) but only keeping the 2 most recent ones (M2, M3) in the serialized state.

Changes committed in 3ee80a1.