Improve command-line completion handling.

[rvuistin]

If you specify the complete name of a command that contains subcommands and options, the expected behavior for completions is:
• Subcommands: All direct subcommands of the specified command should be suggested as completions.
• Options: All options available for that command (including inherited/global options, if any) should also be suggested as completions.

For example, if you have a command structure like:

root
└── parentcmd
    ├── subcmd1
    ├── subcmd2
    └── --option1

When you type:

parentcmd [TAB]

or use the [suggest] directive with parentcmd, the completion list should include:

subcmd1
subcmd2
--option1

Changes:

• enhanced completion logic to handle cases where the text to match is the command name or is null, ensuring all relevant completions (subcommands, options, arguments) are suggested.

• updated CompletionAction in CompletionAction.cs to process multiple tokens for command-line completion, improving support for multi-token inputs.