.NET: Add a ModeProvider for managing agent modes

[westey-m]

Motivation and Context

Description

• Tracks agent mode, between planning and execution.
• Allows both the caller and the agent to switch modes.

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

Introduces a new harness AIContextProvider for tracking an agent “mode” (planning vs execution) in per-session state, exposing tools and helpers to read/change that mode.

Changes:

• Added AgentModeProvider with GetMode/SetMode helpers plus GetMode/SetMode function tools and mode-specific instructions.
• Added AgentModeState stored in AgentSession.StateBag and registered it for source-generated JSON serialization.
• Added unit tests covering default mode, tool behavior, helper behavior, and state persistence.

Reviewed changes

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

File	Description
dotnet/src/Microsoft.Agents.AI/Harness/AgentMode/AgentModeProvider.cs	Adds the provider implementation, instructions, and mode tools backed by session state.
dotnet/src/Microsoft.Agents.AI/Harness/AgentMode/AgentModeState.cs	Adds the persisted session state model for current mode.
dotnet/src/Microsoft.Agents.AI/AgentJsonUtilities.cs	Registers AgentModeState for JSON source generation.
dotnet/tests/Microsoft.Agents.AI.UnitTests/Harness/AgentMode/AgentModeProviderTests.cs	Adds unit tests validating provider/tool/helper behavior and persistence.

[peibekwe]

nit: Does this need to be case-insensitive or we only support lowercase for the values?