[Fix #12873] Fix an error for Metrics/BlockLength

[koic]

Fixes #12873.

This PR fixes an error for Metrics/BlockLength when the CountAsOne config is invalid.

Before

Without the -d option, the cause is unclear:

$ bundle exec rubocop --only Metrics/BlockLength
Inspecting 1 file
An error occurred while Metrics/BlockLength cop was inspecting /Users/koic/src/github.com/koic/rubocop-issues/12873/example.rb:2:2.
To see the complete backtrace run rubocop -d.
.

1 file inspected, no offenses detected

1 error occurred:
An error occurred while Metrics/BlockLength cop was inspecting /Users/koic/src/github.com/koic/rubocop-issues/12873/example.rb:2:2.
Errors are usually caused by RuboCop bugs.
Please, report your problems to RuboCop's issue tracker.
https://github.com/rubocop/rubocop/issues

Mention the following information in the issue report:
1.63.4 (using Parser 3.3.0.5, rubocop-ast 1.31.2, running on ruby 3.3.0) [x86_64-darwin22]

When the -d option is used, hint appears hidden within the backtrace:

$ bundle exec rubocop --only Metrics/BlockLength -d
For /Users/koic/src/github.com/koic/rubocop-issues/12873: configuration from /Users/koic/src/github.com/koic/rubocop-issues/12873/.rubocop.yml
Default configuration from /Users/koic/src/github.com/rubocop/rubocop/config/default.yml
AllCops/Exclude configuration from /Users/koic/src/github.com/koic/rubocop-issues/.rubocop.yml
configuration from /Users/koic/.rbenv/versions/3.3.0/lib/ruby/gems/3.3.0/gems/rubocop-minitest-0.35.0/config/default.yml
configuration from /Users/koic/.rbenv/versions/3.3.0/lib/ruby/gems/3.3.0/gems/rubocop-minitest-0.35.0/config/default.yml
configuration from /Users/koic/src/github.com/rubocop/rubocop-performance/config/default.yml
configuration from /Users/koic/src/github.com/rubocop/rubocop-performance/config/default.yml
configuration from /Users/koic/src/github.com/rubocop/rubocop-rails/config/default.yml
configuration from /Users/koic/src/github.com/rubocop/rubocop-rails/config/default.yml
Use parallel by default.
Skipping parallel inspection: only a single file needs inspection
Inspecting 1 file
Scanning /Users/koic/src/github.com/koic/rubocop-issues/12873/example.rb
An error occurred while Metrics/BlockLength cop was inspecting /Users/koic/src/github.com/koic/rubocop-issues/12873/example.rb:2:2.
Unknown foldable type: :config. Valid foldable types are: array, hash, heredoc, send, csend.
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/metrics/utils/code_length_calculator.rb:54:in `block in build_foldable_checks'

After

Even without the -d option, only the hint is displayed:

$ bundle exec rubocop --only Metrics/BlockLength
Inspecting 1 file
Unknown foldable type: :config. Valid foldable types are: array, hash, heredoc, send, csend.
(from file: /Users/koic/src/github.com/koic/rubocop-issues/12873/example.rb:2:2)
.

1 file inspected, no offenses detected

This behavior is the same as when the Notice config in Style/Copyright cop is not valid. This clarifies what needs to be addressed first to achieve the expected behavior.

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.