Add support for help flag in plugin commands

[hsbt]

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

Fixes #8437

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

Pass --help flag to plugin when using plugin cli.

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

[Copilot]

Pull request overview

Adds plugin-aware help handling so plugin commands can receive a --help request instead of Bundler/Thor crashing when -h/--help is used with a plugin command.

Changes:

• Delegate bundle <plugin_cmd> -h/--help handling to the plugin by intercepting Bundler::CLI#help.
• Add an RSpec covering help-flag behavior for plugin commands.

Reviewed changes

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

File	Description
bundler/lib/bundler/cli.rb	Routes help requests for plugin commands to Bundler::Plugin.exec_command(..., ["--help"]).
bundler/spec/plugins/command_spec.rb	Adds a spec ensuring plugin commands can handle -h/--help without crashing.

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[Copilot]

This spec covers bundle greet --help and bundle greet -h, but the change in Bundler::CLI#help also affects bundle help greet (which is another common way to request help and previously triggered the crash path). Adding an assertion for bundle "help greet" would better lock in the intended behavior and prevent regressions specific to the help subcommand.

[hsbt]

@copilot open a new pull request to apply changes based on this feedback

[Copilot]

@hsbt I've opened a new pull request, #9265, to work on those changes. Once the pull request is ready, I'll request review from you.