Add InvocationRequired to FunctionCallContent; make MCP types inherit from base content types

[Copilot]

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.

Core Changes

• FunctionCallContent: Unsealed; added InvocationRequired property (defaults to true)
• FunctionResultContent: Unsealed
• McpServerToolCallContent: Now extends FunctionCallContent, sets InvocationRequired = false since MCP calls are handled by the service
• McpServerToolResultContent: Now extends FunctionResultContent, Output property replaced by inherited Result

FunctionInvokingChatClient

• Skips function calls where InvocationRequired == false
• Sets InvocationRequired = false after processing a function call
• Excludes McpServerToolCallContent from approval request/response processing (handled externally)

OpenAI Client

• Uses FunctionApprovalRequestContent with embedded McpServerToolCallContent instead of McpServerToolApprovalRequestContent
• Uses FunctionApprovalResponseContent instead of McpServerToolApprovalResponseContent

Example

// MCP tool calls from OpenAI are automatically skipped by FunctionInvokingChatClient
var mcpCall = new McpServerToolCallContent("callId", "toolName", "serverName");
Assert.False(mcpCall.InvocationRequired); // Already handled by service

// Regular function calls still need processing
var functionCall = new FunctionCallContent("callId", "funcName");
Assert.True(functionCall.InvocationRequired);

// After FunctionInvokingChatClient processes it:
functionCall.InvocationRequired = false; // Won't be processed again

Original prompt

Can you create a new PR based off #7201 and also implementing #7126 (even though is closed); the McpServerToolCallContent and McpServerToolResultContent would always set InvocationRequired = false since they represent calls that are handled by the service.

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