Avoid mutable global state

[tmat]

The implementation of IsDynamic extension property mutated global state and was not thread-safe.
This could cause state corruption when commands are created in parallel, e.g. in unit tests.

Removes global state and instead defines ExtendedArgument<T> and ExtendedOption<T> that allow setting the flag on select symbols.

[tmat]

@MiYanni ptal

[baronfel]

Please don't do this - we can't use subclasses to contain behavior because it introduces diamond dependencies, and we have several permutations of cross cutting S.CL extensibility behavior.

[tmat]

Which other extensions do we have?

[Copilot]

Pull request overview

This PR addresses a thread-safety issue in System.CommandLine.StaticCompletions where IsDynamic metadata was tracked via mutable global state. It replaces that approach with explicit symbol types (ExtendedArgument<T>, ExtendedOption<T>) that carry the IsDynamic flag on the symbol instance.

Changes:

• Added IExtendedSymbol plus ExtendedOption<T> / ExtendedArgument<T> to hold IsDynamic per symbol instance.
• Removed the prior DynamicSymbolExtensions implementation that used a global dictionary for dynamic symbol state.
• Updated several CLI option/argument factories to construct ExtendedOption<T> / ExtendedArgument<T>.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/System.CommandLine.StaticCompletions/shells/PowershellShellProvider.cs	Updates dynamic-argument detection to use IExtendedSymbol rather than the old extension property.
src/System.CommandLine.StaticCompletions/ExtendedSymbols.cs	Introduces IExtendedSymbol, ExtendedOption<T>, ExtendedArgument<T>, and a Symbol IsDynamic extension getter.
src/System.CommandLine.StaticCompletions/DynamicSymbolExtensions.cs	Removes the previous global-dictionary-based IsDynamic implementation.
src/Cli/Microsoft.DotNet.Cli.Definitions/Common/TargetPlatformOptions.cs	Switches runtime option creation to ExtendedOption<string>.
src/Cli/Microsoft.DotNet.Cli.Definitions/Common/CommonOptions.cs	Switches several common options to ExtendedOption<T>.
src/Cli/Microsoft.DotNet.Cli.Definitions/Common/CommonArguments.cs	Switches package identity arguments to ExtendedArgument<T>.
src/Cli/Microsoft.DotNet.Cli.Definitions/Commands/Restore/RestoreCommandDefinition.cs	Switches restore runtime option creation to ExtendedOption<IEnumerable<string>>.
src/Cli/Microsoft.DotNet.Cli.Definitions/Commands/Reference/ReferenceAddCommandDefinition.cs	Switches framework option creation to ExtendedOption<string>.
src/Cli/Microsoft.DotNet.Cli.Definitions/Commands/Package/PackageAddCommandDefinition.cs	Switches version option creation to ExtendedOption<string>.

[Copilot]

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[baronfel]

Which other extensions do we have?

We've got the forwarding stuff at minimum, and help descriptions for the same symbol per-command IIRC?

[baronfel]

Closing because I merged #52887