Further refactor use of SafeMarkup in HWLDFWordAccumulator

Problem/Motivation

Follow-up to #2280963: Refactor use of SafeMarkup in HWLDFWordAccumulator. In that issue, a combination of SafeMarkup::checkPlain() and SafeMarkup::set() were converted to SafeMarkup::format(). This makes sense. However, the added use of SafeMarkup::format() is not best practice because it passes a variable as the first argument:

$this->line = SafeMarkup::format($format_string, ['@original_line' => $this->line, '@group' => $this->group]);

This is not technically incorrect because $format_string is defined as one or the other of two string literals in an if/else immediately above and is composed of @ tokens (so there is no chance of unsanitized user input in the variable), and since it's not a translated string, we also don't need to worry about scanning for the translations. However, the code would set a better example (and be easier to read and security audit) if SafeMarkup::format were just used with the appropriate pattern in the two different codepaths.

Proposed resolution

Use SafeMarkup::format with the appropriate pattern in the two different codepaths. Patch attached.

Beta phase evaluation

Remaining tasks

Needs review.

User interface changes

N/A

API changes

N/A