.NET: Implement tool events > function call events for AG-UI support

[kzu]

Added two new case branches and three new converter methods:

• ToolExecutionStartEvent → FunctionCallContent (the AGUI layer then emits TOOL_CALL_START/ARGS/END to the client)
• ToolExecutionCompleteEvent → FunctionResultContent → TOOL_CALL_RESULT
• Arguments are converted from JsonElement using AOT-safe JsonElement.EnumerateObject() — no reflection/dynamic code

Motivation and Context

When using Copilot SDK adapter to expose an AG-UI client, tool call events aren't surfaced at all as events the client can show.

Fixes #4823

[Copilot]

Pull request overview

Adds forwarding of GitHub Copilot SDK tool execution events into the Agent Framework’s function-call/function-result content model so the AG-UI adapter can surface tool-call lifecycle events to clients (fixing #4823).

Changes:

• Handle ToolExecutionStartEvent and ToolExecutionCompleteEvent in the Copilot session streaming event switch.
• Convert tool-start to FunctionCallContent (including JSON argument conversion) and tool-complete to FunctionResultContent.
• Add unit tests covering tool start/complete conversions and JSON argument handling.

Reviewed changes

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

File	Description
dotnet/src/Microsoft.Agents.AI.GitHub.Copilot/GitHubCopilotAgent.cs	Adds streaming handling + conversion helpers to emit FunctionCallContent/FunctionResultContent from tool events.
dotnet/tests/Microsoft.Agents.AI.GitHub.Copilot.UnitTests/GitHubCopilotAgentTests.cs	Adds tests validating tool event conversion behavior and argument mapping.

[Copilot]

ConvertJsonElementToArguments currently converts non-primitive argument values (e.g., JsonValueKind.Object/Array) using GetRawText(), which turns them into JSON strings. When these arguments are later serialized (e.g., into AG-UI ToolCallArgs events), nested objects/arrays will be double-encoded instead of remaining structured JSON. Prefer preserving structured values by returning JsonElement (e.g., property.Value.Clone()) for Object/Array (and consider how to handle Undefined), so downstream serialization emits proper JSON objects/arrays.

Suggested change

	: property.Value.GetDouble(),
	: property.Value.GetDouble(),
	JsonValueKind.Object => property.Value.Clone(),
	JsonValueKind.Array => property.Value.Clone(),
	JsonValueKind.Undefined => null,

[Copilot]

The test currently asserts objVal is a string (raw JSON text). If arguments are intended to round-trip as structured JSON through the AG-UI layer, nested object/array values should be preserved as structured types (e.g., JsonElement) rather than raw JSON strings. Consider updating this assertion to validate the intended structured representation so regressions don’t reintroduce double-encoding of nested arguments.

Suggested change

	// Non-primitive values fall back to raw JSON text
	Assert.IsType<string>(content.Arguments["objVal"]);
	// Non-primitive values are preserved as structured JSON
	var objValElement = Assert.IsType<JsonElement>(content.Arguments["objVal"]);
	Assert.Equal("value", objValElement.GetProperty("nested").GetString());