Fix missing columns in Markdown tables without outer pipes

[unknowlei]

A table like this currently shows only the Revenue column, with Acme under it and the numbers missing:

Name | Revenue | Profit
--- | --- | ---
Acme | 100 | 20

The header parser was always dropping the first and last split cells. This only removes them when the line actually starts or ends with a pipe, so tables with either, both, or neither outer pipe keep their columns.

I added seven focused tests covering those four forms and empty header cells, including the rendered output. bun test ./src/utils/markdown-table.test.ts passes; the three cases with missing outer pipes failed before the fix.