fix: parse flags for help subcommand (#2271)

[barry3406]

What type of PR is this?

• bug

What this PR does / why we need it:

PR #2245 introduced a regression in v3.6.2 by short-circuiting flag pre-parsing for any command named `help`:

if cmd.isCompletionCommand || cmd.Name == helpName {
    cmd.parsedArgs = args
    return ctx, cmd.Action(ctx, cmd)
}

The original goal of #2245 was to let `help` and `completion` run without tripping required-flag enforcement, but skipping pre-parse entirely also drops flags and arguments on user-supplied help commands (#2271) and breaks custom help commands (#2260).

This PR keeps the pre-parse path intact for the help command and instead short-circuits `checkAllRequiredFlags` for `help` and completion, which was the actual intent. The completion command still bypasses pre-parse as it did before #2245.

• `command_run.go`: restore the pre-Fix:(issue_2244) Dont check req flags for help and completion commands #2245 condition (`isCompletionCommand` only).
• `command.go`: exempt `help` and completion commands from the required-flag check.

Which issue(s) this PR fixes:

Fixes #2271

Testing

Added `Test_HelpCommand_ParsesFlags` in `help_test.go`, which fails on current main and passes with this change. The existing `Test_HelpCommand_RequiredFlagsNoDefault` added in #2245 still passes.

```
make lint vet test
```

Release Notes

```release-note
Fix v3.6.2 regression where flags and arguments on user-supplied help subcommands were silently dropped.
```

[aryounce]

Thanks for a concise fix.