fix(custom-tags): restructure inline content with newlines to prevent ATX heading block splitting

[sleitor]

Problem

Custom tags with multiline content are prematurely closed by the CommonMark parser when the content starts inline (on the same line as the opening tag) and contains a newline followed by a block-level element like an ATX heading.

Minimal reproduction:

const markdown = '<ai-thinking> hi\n # yes</ai-thinking>';
// Expected: Both 'hi' and '# yes' inside the AiThinking component
// Actual: 'hi' inside, '# yes' rendered outside

The marked Lexer splits this as:

1. paragraph: <ai-thinking> hi\n
2. heading: # yes</ai-thinking>

Root Cause

preprocess-custom-tags.ts only restructures tags to block-level HTML when there are blank lines (\n\n) in the content. However, a single newline inside inline content is sufficient to introduce block-starting sequences (ATX headings, thematic breaks, list items, etc.) that break the parser.

Content that already starts on its own line (\nContent\n) is correctly parsed as a block-level HTML element by the Lexer — even with headings inside — because the opening tag is on its own line.

Fix

Extend the restructuring condition to also apply when:

• Content is inline (doesn't start with \n), AND
• Content contains any newline

Tags where content already starts on its own line (block-level) are unaffected. Single-line tags without newlines are also unaffected.

Fixes #477
Related: #478 (same root cause)

[vercel]

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