Added .preheader-spacing plaintext exclusion

[sam-lord]

ref ONC-1682

The spacing inserts a bunch of dodgy characters into the plaintext version of emails.

[coderabbitai]

Walkthrough

This PR modifies the HTML-to-plaintext conversion pipeline in the TryGhost SDK to skip an additional CSS selector during email generation. The emailSettings configuration in loadConverters() now excludes both .preheader and .preheader-spacing elements when converting HTML emails to plaintext format, preventing these preheader-related sections from appearing in the text output.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The PR title accurately and specifically describes the main change: adding .preheader-spacing to plaintext exclusions for email conversion.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The PR description relates to the changeset by explaining the problem (spacing causing encoding artifacts in plaintext emails) and the solution (adding .preheader-spacing exclusion), matching the code changes to skip this element.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix-preheader-spacing-in-emails

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.96%. Comparing base (defb53e) to head (03ead63).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #912   +/-   ##
=======================================
  Coverage   99.96%   99.96%           
=======================================
  Files          33       33           
  Lines        3072     3073    +1     
  Branches      477      477           
=======================================
+ Hits         3071     3072    +1     
  Misses          1        1           

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