refactor(mdxish): extend table types to reflect flow-content cells

[eaglethrost]

🎫 Follow-up to #1445 (RM-16217)

🎯 What does this PR do?

Follow-up on #1445 to improve the typing around tables. The issue was mdast's built-in TableCell is typed as PhrasingContent[], but our pipeline processes and put block-level nodes (paragraphs, lists, code) inside cells so they can serialize to JSX <Table> with proper paragraph separation. The mismatch meant the tests and transformers leaned on as Table / as TableCell casts to force the data through. Nothing was wrong at runtime, but good to clean up so the types describe what actually flows through and call sites don't need unsafe casts.

Solution is to add a small set of mdxish-specific table types (MdxishTable, MdxishTableRow, MdxishTableCell, MdxishMdastRoot) that widen TableCell.children to the content the serializer actually accepts. The transformers now return/consume these shapes, and the test fixtures use MdxishMdastRoot instead of casting each table literal.

• New types in processor/transform/mdxish/types.ts + MdxishMdastRoot re-exported from types.d.ts
• mdxish-jsx-to-mdast.ts and tables/mdxish-tables-to-jsx.ts use the wider shapes; no more as TableCell / as Table at cell pushes
• __tests__/lib/mdxish/mdxishMdastToMd.test.ts typed as MdxishMdastRoot — all 14 as Table casts gone

🧪 QA tips

• `npm test` — no behavior change; existing snapshots should still match exactly
• Spot-check that tables with paragraph/code/list content in cells still serialize to JSX `` correctly

[eaglethrost]

I've put the Root extension interface here in case we want to use it in a consumer. Since this is a superset of Root, we don't need to use it, but of course it would be better typing for the Table serialisation / deserialisation code.