Allow @Spec on method parameters for method-based subcommands

[EmperorSARATH]

This PR addresses issue #1670 by allowing @Spec to be used on method parameters in method-based subcommands.

Changes:

• Added ElementType.PARAMETER to @Spec to enable usage on method parameters.
• Inject the correct CommandSpec via commandMethodParamValues() for method subcommands.
• Prevent @Spec parameters from being treated as CLI arguments.
• Scoped the change to method parameters to avoid affecting existing behavior.
• Verified that all existing tests pass.

Motivation:

Previously, method-based subcommands could not directly receive a CommandSpec via a parameter, forcing developers to use fragile workarounds like looking up the subcommand from the parent spec. This PR provides a clean and safe way to inject the subcommand's CommandSpec.

Example usage:

import picocli.CommandLine;
import picocli.CommandLine.Command;
import picocli.CommandLine.Spec;
import picocli.CommandLine.Model.CommandSpec;

@Command(name = "app", subcommands = {TestApp.class})
public class TestApp {

    public static void main(String[] args) {
        CommandLine cmd = new CommandLine(new TestApp());
        cmd.execute(args);
    }

    @Command(name = "sub")
    void sub(@Spec CommandSpec spec) {
        System.out.println("Subcommand name: " + spec.name());
    }
}

Example run :

$ java -cp .:build/libs/picocli-4.7.8-SNAPSHOT.jar TestApp sub
Subcommand name: sub

Fixes: #1670