Fix Lint/Syntax behavior when Enabled: false of Lint department

[koic]

Here's another fix I found in work on #11688.

Ruby code has a syntax error and .rubocop.yml with Lint department is disabled:

def foobar
  t = {
    a: "foo"  # missing "," syntax error
    b: 'bar'
  }
  return t
end

# .rubocop.yml
Lint:
  Enabled: false

But Lint/Syntax must not be disabled:

There is one exception from the general rule above and that is Lint/Syntax, a special cop that checks for syntax errors before the other cops are invoked. It cannot be disabled and its severity (fatal) cannot be changed in configuration.

https://docs.rubocop.org/rubocop/1.46/configuration.html#severity

Before

The syntax is invalid but an offense is not registerd:

$ bundle exec rubocop
(snip)

Inspecting 1 file
.

1 file inspected, no offenses detected

After

Lint/Syntax detects invalid syntax as expected:

$ bundle exec rubocop
(snip)

Offenses:

example.rb:4:5: F: Lint/Syntax: unexpected token tIDENTIFIER
(Using Ruby 3.2 parser; configure using TargetRubyVersion parameter, under AllCops)
    b: 'bar'
    ^

1 file inspected, 1 offense detected

The Lint/Syntax behavior for DisabledByDefault: true configuration is also fixed.

---

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• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
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• Added tests.
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[bbatsov]

Nice catch!