Fix multi-line opening tags placing closing tags on own line

[jlevy]

Summary

• When Flowmark wraps long Markdoc tags across multiple lines, the closing tag could end up on the same line as the opening tag's closing delimiter (%}), triggering a Markdoc parser bug
• This fix ensures that when an opening tag spans multiple lines, the closing tag is placed on its own line

Before (triggers Markdoc bug):

{% field kind="string" id="name" label="Name" required=true
maxLength=50 %}{% /field %}

After (correct):

{% field kind="string" id="name" label="Name" required=true
maxLength=50 %}
{% /field %}

Changes

• Added _fix_multiline_opening_tag_with_closing() function in tag_handling.py
• Integrated fix into all wrapper return paths in add_tag_newline_handling()
• Added 4 unit tests in test_tag_formatting.py
• Added Issue 7 test cases in testdoc

Test plan

• make lint passes
• make test passes (160 tests)
• Verified with reproduction case from issue Line wrapping inside Markdoc tags breaks parser when closing tag is on same line #17

Fixes #17

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