A handful of S.CL-related cleanups and reorgs from the OTel branch to minimize diffs

[baronfel]

This slices off a few S.CL usability refactors off of #49409 to minimize the diff on that monstrosity.

This set of changes

• leans into S.CL's Action pipeline for cross-cutting options that have behaviors attached to them
• re-organizes Program.cs into sub-methods that have specific intents for easier reading
• removes some unused S.CL extension methods
• has a little bit of nullability-adoption that went along with that stuff

[Copilot]

Pull Request Overview

This PR refactors the dotnet CLI to improve System.CommandLine (S.CL) usage patterns and code organization. The changes prepare the codebase for OpenTelemetry integration by removing performance measurement handling from the main command processing flow and adopting S.CL's Action pipeline for cross-cutting behaviors.

Key Changes:

• Reorganizes Program.cs into focused sub-methods for better readability and maintainability
• Adopts S.CL's Action pipeline for options like --version, --info, and --diagnostic that have behaviors attached
• Removes performance measurement tracking from telemetry tests and command processing
• Adopts nullable reference types across several files to improve type safety

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/dotnet.Tests/TelemetryCommandTest.cs	Removes performance measurement testing and simplifies test assertions
src/Cli/dotnet/Telemetry/Telemetry.cs	Adopts nullable reference types for telemetry properties and parameters
src/Cli/dotnet/Program.cs	Major reorganization into focused methods with nullable adoption and Activity-based tracing
src/Cli/dotnet/PerformanceLogEventSource.cs	Adopts nullable reference types for startup information parameters
src/Cli/dotnet/Parser.cs	Implements S.CL Action classes for --version and --info options
src/Cli/dotnet/Extensions/ParseResultExtensions.cs	Simplifies extension methods and removes unused functionality
src/Cli/dotnet/CommonOptionsFactory.cs	Implements S.CL Action for diagnostic mode option
src/Cli/Microsoft.DotNet.Cli.Utils/Activities.cs	Adds tracing constants for cross-process activity tracking