Add content to represent service-side actions

[jozkee]

Introduce ServiceActionContent and implement it in CodeInterpreter and McpServerTool contents.

Naming considerations:

• HostedServiceCallContent - Not nice for Result contents, and may be confusing with FunctionCallContent.
• HostedServiceSideContent - "Side" is uncommon in the codebase.
• HostedServiceContent - If we use this name, HostedFileContent should also extend it which escapes the scope of "content about server side actions".
• ServiceActionContent - Breaks the Hosted prefix we've been using but I like the shortness; con: "Action" may be too broad.

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR introduces ServiceActionContent as a new base class for content types representing service-side actions. It refactors CodeInterpreterToolCallContent, CodeInterpreterToolResultContent, McpServerToolCallContent, and McpServerToolResultContent to extend this base class, consolidating the common Id property (previously CallId). This change improves the type hierarchy and provides a cleaner abstraction for service-hosted operations.

Key Changes

• Introduces ServiceActionContent base class with a required Id property
• Updates code interpreter and MCP server tool content classes to inherit from ServiceActionContent
• Standardizes property naming from CallId to Id across all service action content types
• Makes id a required constructor parameter (previously nullable/optional)

Reviewed changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/ServiceActionContent.cs	New base class for service-side actions with required Id property
src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/CodeInterpreterToolCallContent.cs	Updated to extend ServiceActionContent and require id in constructor
src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/CodeInterpreterToolResultContent.cs	Updated to extend ServiceActionContent and require id in constructor
src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/McpServerToolCallContent.cs	Updated to extend ServiceActionContent, removing duplicate CallId property
src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/McpServerToolResultContent.cs	Updated to extend ServiceActionContent, removing duplicate CallId property
src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/AIContent.cs	Added ServiceActionContent to polymorphic type comments
src/Libraries/Microsoft.Extensions.AI.Abstractions/Utilities/AIJsonUtilities.Defaults.cs	Registered ServiceActionContent for JSON serialization
src/Libraries/Microsoft.Extensions.AI.Abstractions/ChatCompletion/ChatResponseExtensions.cs	Updated coalescing logic to use Id property
src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIResponsesChatClient.cs	Updated to use Id property and new constructors
src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIAssistantsChatClient.cs	Updated to use new constructors with required id
test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Contents/ServiceActionContentTests.cs	New comprehensive test suite for ServiceActionContent
test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Contents/CodeInterpreterToolCallContentTests.cs	Updated tests for constructor changes and added serialization test
test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Contents/CodeInterpreterToolResultContentTests.cs	Updated tests for constructor changes and added validation test
test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Contents/McpServerToolCallContentTests.cs	Updated tests for property renaming and added serialization test
test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Contents/McpServerToolResultContentTests.cs	Updated tests for property renaming
test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Contents/AIContentTests.cs	Added ServiceActionContent to polymorphic serialization test
test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIResponseClientTests.cs	Updated integration tests to use Id property
test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIResponseClientIntegrationTests.cs	Updated integration tests to use Id property
test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIAssistantChatClientIntegrationTests.cs	Simplified assertions to use required Id property