[2.x] Fix Markdown heading conversion

[emmadesilva]

What changed

• Narrow the ATX heading regex so it removes Markdown heading markers without consuming unrelated content across lines.
• Add regression coverage for long Markdown documents containing headings followed by wide tables.
• Update the empty-line expectation to reflect preservation of the remaining line breaks.

Why

The previous regex used optional newline and unbounded whitespace matching. On long documents, it could consume content beyond the heading markers and corrupt the plain-text result. Restricting whitespace to spaces and tabs keeps matching local to each heading line.

Impact

Markdown-to-plain-text conversion now preserves content following ATX headings, including wide tables, while continuing to remove opening and closing heading markers.

Validation

• vendor/bin/phpunit packages/framework/tests/Feature/Actions/ConvertsMarkdownToPlainTextTest.php (29 tests, 31 assertions)
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (44b4b56) to head (b69b3af).
⚠️ Report is 3 commits behind head on 2.x.

Additional details and impacted files

@@ Coverage Diff @@## 2.x #2590 +/- ##
===========================================
Coverage 100.00% 100.00% Complexity 1615 1615 ===========================================
Files 169 169 Lines 4074 4074 ===========================================
Hits 4074 4074 

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[Copilot]

Pull request overview

Narrows ATX heading matching to avoid consuming content across lines during Markdown-to-text conversion.

Changes:

• Restricts heading whitespace matching to spaces and tabs.
• Adds regression coverage for headings followed by wide tables.
• Updates empty-line preservation expectations.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
ConvertsMarkdownToPlainText.php	Refines ATX heading conversion.
ConvertsMarkdownToPlainTextTest.php	Adds regression coverage and updates newline expectations.

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