fix: handle dom tables properly when copy pasting

[pepefeliblu]

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Summary

This PR addresses an issue where copying and pasting DOM tables (e.g., from web pages) into Milkdown resulted in plain text or paragraphs instead of Markdown tables. #2159
The root cause was that the parseDOM rule for table_header_row in @milkdown/preset-gfm was too strict, requiring a data-is-header attribute. Standard HTML tables typically use <tr> elements containing <th> children for headers, without specific data attributes.

Changes:

• Updated packages/plugins/preset-gfm/src/node/table/schema.ts to include a new parseDOM rule for table_header_row. This rule identifies a <tr> as a header row if it contains any <th> elements.
• Refactored table node schemas to export their spec factories (e.g., tableSpec, tableHeaderRowSpec) to enable better unit testing of the schema definitions.

How did you test this change?

I verified the fix by creating a local temporary reproduction test and running the full end-to-end regression suite.

1. Local Reproduction Test (Verified & Cleaned up):
   I created a temporary test case to parse a standard HTML table string:

   <table>
     <tr>
       <th>Header 1</th>
       <th>Header 2</th>
     </tr>
     <tr>
       <td>Cell 1</td>
       <td>Cell 2</td>
     </tr>
   </table>

   Result: The parser correctly identified the structure as a table containing table_header_row and table_row.
2. Regression Testing:
   Ran the full e2e test suite to ensure no existing functionality was broken.
   Command: pnpm --filter=@milkdown/e2e test
   Result: All 97 tests passed.

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commit: 3b34b4b