Add test coverage for bundle help <plugin_command>

[Copilot]

What was the end-user or developer problem that led to this PR?

The test suite for plugin command help support only covered bundle greet --help and bundle greet -h, but did not test bundle help greet. This is a common help invocation pattern that could regress without test coverage.

What is your fix for the problem, implemented in this PR?

Added a third assertion to verify bundle help greet works correctly with plugin commands:

bundle "help greet"
expect(out).to eq("Usage: bundle greet [NAME]")

This exercises the code path in Bundler::CLI#help that detects plugin commands and delegates to Bundler::Plugin.exec_command(cli, ["--help"]).

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

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[Copilot]

Pull request overview

This PR adds test coverage for the bundle help <plugin_command> invocation pattern to ensure plugin commands work correctly with the help system.

Changes:

• Added test assertion for bundle help greet to verify it produces the same output as bundle greet --help and bundle greet -h

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