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.

[rvuistin]

I am closing this pull request because the proposed change is not necessary.
After further review, I realized that suggestions for subcommands only appear after typing an additional space, which is the expected behavior.
Therefore, the motivation for this PR is not justified.
Thank you for your understanding.