[Nitro CLI] Properly handle invalid --output option value

[tobias-tengler]

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[Copilot]

Pull request overview

This PR updates Nitro CLI command execution to properly handle command-line parse errors (specifically invalid --output values) by short-circuiting execution before session/client initialization, and adds/adjusts tests to cover the behavior.

Changes:

• Short-circuit NitroRootCommand.ExecuteAsync(...) on parse errors and delegate to ParseResult.InvokeAsync(...) for correct error reporting/exit codes.
• Add a command-level test verifying invalid --output values return an error and the expected parse error output.
• Consolidate/extend environment-variable default option testing and introduce shared test utilities (fixture, interaction mode enum, error factory).

Reviewed changes

Copilot reviewed 5 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Nitro/CommandLine/src/CommandLine/Extensions/RootCommandExtensions.cs	Avoids accessing parsed option values when parsing failed; relies on System.CommandLine’s error reporting.
src/Nitro/CommandLine/test/CommandLine.Tests/Commands/GlobalOptionsCommandTests.cs	Adds regression test for invalid --output value handling.
src/Nitro/CommandLine/test/CommandLine.Tests/GlobalOptionsTests.cs	Moves/extends env var default tests and refactors the test harness to support env provider injection/probe command configuration.
src/Nitro/CommandLine/test/CommandLine.Tests/Options/EnvironmentVariableDefaultTests.cs	Removes now-redundant standalone env var default test file.
src/Nitro/CommandLine/test/CommandLine.Tests/Commands/NitroCommandFixture.cs	Introduces shared root-command fixture for command tests.
src/Nitro/CommandLine/test/CommandLine.Tests/Commands/InteractionMode.cs	Adds interaction mode enum used across command tests.
src/Nitro/CommandLine/test/CommandLine.Tests/Commands/MockErrorFactory.cs	Adds centralized mock error setup helpers used by multiple command test bases.
src/Nitro/CommandLine/test/CommandLine.Tests/Commands/README.md	Removes the command test migration guide document.

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