fix: render markdown tables correctly in chat responses

[ajayjha1]

Description

Fix markdown tables rendering as unformatted text in chat responses.

Two issues were causing this:

1. In remarkTables.tsx, header cells were built with type: "element", tagName: "th" — a raw hast node injected into an MDAST tree. The remark-to-rehype pipeline doesn't handle this correctly, causing header cells to fall through as plain text. Changed to type: "tableCell" to match how body cells are built.
2. In StyledMarkdownPreview/index.tsx, the StyledMarkdown component had no table, th, or td CSS rules. Table styles in markdown.css are scoped to .wmde-markdown which is never applied here, so tables had no borders or styling. Added table styles using VS Code CSS variables for theme compatibility.

Fixes #12167

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Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Tests

No new tests added. The existing remarkTables plugin logic is covered by the change to use the correct MDAST node type (tableCell), which is consistent with how body rows were already being built. Manual testing confirmed tables now render with proper borders, bold headers, and alternating row colors in the VS Code extension chat.

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Summary by cubic

Fixes markdown tables rendering as plain text in chat responses. Header cells now parse correctly, and tables use theme-aware styles with borders and alternating rows.

• Bug Fixes
  • Changed header cells from a raw hast element th to MDAST tableCell in the remark-to-rehype pipeline so headers render properly.
  • Added CSS for table, th, and td in StyledMarkdown using VS Code variables for borders, bold headers, and zebra striping.

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[cubic-dev-ai]

No issues found across 2 files

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