Render bullet lists as proper ProseMirror nodes in from_markdown()

[yf-chau]

Summary

from_markdown() strips bullet markers (* / -) but emits each item as an independent paragraph node instead of a bullet_list containing list_item children. This means <li> tags are rendered as plain text paragraphs rather than as a formatted list on Substack.

Before (current behavior)

Each bullet becomes a flat paragraph — <li> elements are lost:

{"type": "paragraph", "content": [{"type": "text", "text": "Item 1"}]},
{"type": "paragraph", "content": [{"type": "text", "text": "Item 2"}]},
{"type": "paragraph", "content": [{"type": "text", "text": "Item 3"}]}

After (this PR)

Consecutive bullets are grouped into a proper bullet_list node, producing correct <ul><li> markup:

{
  "type": "bullet_list",
  "content": [
    {"type": "list_item", "content": [{"type": "paragraph", "content": [{"type": "text", "text": "Item 1"}]}]},
    {"type": "list_item", "content": [{"type": "paragraph", "content": [{"type": "text", "text": "Item 2"}]}]},
    {"type": "list_item", "content": [{"type": "paragraph", "content": [{"type": "text", "text": "Item 3"}]}]}
  ]
}

Non-bullet lines and blank lines flush the pending list, so mixed content (paragraphs interspersed with lists) works correctly.

🤖 Generated with Claude Code