Added Shell tool abstractions

[dmytrostruk]

Added shell tool abstractions to Microsoft.Extensions.AI.Abstractions for representing shell tool interactions with AI services.

New types:

• HostedShellTool : AITool — marker tool for hosted shell execution by a service
• ShellTool : AIFunction (abstract) — base class for local shell execution, subclassed by users to provide InvokeCoreAsync
• ShellCallContent : FunctionCallContent — represents a shell tool call from a service
• ShellResultContent : FunctionResultContent — represents the result of a shell tool invocation
• ShellCommandOutput — structured output of a single command (stdout, stderr, exit code, timeout)

Registered ShellCallContent and ShellResultContent for polymorphic JSON serialization. All types are marked [Experimental].

Microsoft Reviewers: Open in CodeFlow

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[Copilot]

Pull request overview

This PR adds shell tool abstractions to Microsoft.Extensions.AI.Abstractions, enabling AI services to interact with shell commands through both hosted (service-executed) and local (client-executed) patterns. The implementation follows established patterns from similar features like image generation and code interpreter tools.

Changes:

• Added five new experimental types for shell tool interactions: HostedShellTool (marker for service-side execution), ShellTool (abstract base for client-side execution), ShellCallContent (represents shell call requests), ShellResultContent (represents execution results), and ShellCommandOutput (structured output data)
• Registered ShellCallContent and ShellResultContent for polymorphic JSON serialization in AIJsonUtilities
• Added AIShell experimental diagnostic ID to enable [Experimental] marking of all shell-related types

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Shared/DiagnosticIds/DiagnosticIds.cs	Added AIShell experimental diagnostic ID
src/Libraries/Microsoft.Extensions.AI.Abstractions/Utilities/AIJsonUtilities.Defaults.cs	Registered ShellCallContent and ShellResultContent for polymorphic serialization with type discriminators "shellCall" and "shellResult"
src/Libraries/Microsoft.Extensions.AI.Abstractions/Tools/ShellTool.cs	Abstract base class for client-side shell execution, extends AIFunction with fixed name "local_shell"
src/Libraries/Microsoft.Extensions.AI.Abstractions/Tools/HostedShellTool.cs	Marker tool for service-side shell execution with name "shell", extends AITool
src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/ShellResultContent.cs	Represents shell execution results, extends FunctionResultContent with Output and MaxOutputLength properties
src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/ShellCommandOutput.cs	Data class for single command output (stdout, stderr, exit code, timeout flag)
src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/ShellCallContent.cs	Represents shell call requests, extends FunctionCallContent with Commands, TimeoutMs, MaxOutputLength, and Status properties
src/Libraries/Microsoft.Extensions.AI.Abstractions/Contents/AIContent.cs	Added commented JsonDerivedType entries for ShellCallContent and ShellResultContent (to be uncommented when no longer experimental)
test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Tools/ShellToolTests.cs	Tests for ShellTool including constructor, properties, and InvokeAsync with ShellResultContent return
test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Tools/HostedShellToolTests.cs	Tests for HostedShellTool constructor and AdditionalProperties handling
test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Contents/ShellResultContentTests.cs	Comprehensive tests including properties, multiple outputs, and polymorphic serialization
test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Contents/ShellCommandOutputTests.cs	Tests for ShellCommandOutput properties and serialization including timeout scenarios
test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Contents/ShellCallContentTests.cs	Tests for ShellCallContent including constructor, properties, and polymorphic serialization

[stephentoub]

Thanks, @dmytrostruk. We'll want to hold on this until OpenAI 2.9.0 is out and we can better validate the design with the shell tool support it exposes, implementing support for these types in Microsoft.Extensions.AI.OpenAI.

[stephentoub]

What is the use of this if there are no concrete implementations? Who is expected to provide derived implementations?

[stephentoub]

Assuming we want this property, it should be a TimeSpan named Timeout