[WIP] Update MCP server tool invocation handling

[Copilot]

This PR combines work from #7201 and #7126.

Summary

McpServerToolCallContent and McpServerToolResultContent now extend FunctionCallContent and FunctionResultContent respectively. The new InvocationRequired property (set to false by default for MCP content) prevents FunctionInvokingChatClient from re-processing function calls that have already been handled by the hosted MCP service.

Changes

• McpServerToolCallContent now extends FunctionCallContent instead of AIContent
• McpServerToolResultContent now extends FunctionResultContent instead of AIContent
• New InvocationRequired property is set to false by default for MCP content
• Removed McpServerToolApprovalRequestContent and McpServerToolApprovalResponseContent (no longer needed)
• Updated serialization to use IDictionary<string, object?> instead of IReadOnlyDictionary<string, object?>
• Added CompatibilitySuppressions.xml to suppress expected API breaking changes
• Fixed tests: replaced .ToolName with .Name where needed
• Removed stale comment in UserInputResponseContentTests.cs
• Build passes
• Tests pass

Original prompt

Open PR based off of copilot/update-mcp-server-tool-invocation.

The branch combines work from #7201 and #7126. McpServerToolCallContent and McpServerToolResultContent now extend FunctionCallContent and FunctionResultContent respectively. The new InvocationRequired property prevents FunctionInvokingChatClient from re-processing function calls that have already been handled.

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