fix(bash): heredoc regex should not match pipe at end of line

[xxiaoxiong]

Fixes #4377

Problem

When a here-document is followed by a pipe (e.g., echo "ok" <<-EOF | cat), the current heredoc begin regex (<<-?\s*(\w+)\b.*) matches the entire rest of the line, causing | cat to be incorrectly highlighted as part of the heredoc string (scope: string), so cat does not get recognized as a built_in.

Fix

Add a negative lookahead (?!\s*\|) to the heredoc begin regex. This prevents the heredoc from being triggered when a pipe operator follows the delimiter keyword, allowing pipe-separated commands to be highlighted correctly.

Before (rendered HTML)

echo "ok" <span class="hljs-string">&lt;&lt;-EOF | cat
\tHello
EOF</span>

After (rendered HTML)

echo "ok" &lt;&lt;-EOF | <span class="hljs-built_in">cat</span>
\tHello
EOF

Test

• Added heredoc-redirect markup test covering heredoc followed by pipe
• Existing strings markup test updated for improved rendering behavior
• All 539 markup tests pass (the only failure is a pre-existing python test unrelated to this change)