fix(jupyter): report codemirror_mode as a string (#36241)
The Jupyter `kernel_info_reply` reported `codemirror_mode` as an object
`{ name: "typescript" }` that omitted a `version` field. Consumers that
model `codemirror_mode` as either a bare string or an object with a
required numeric `version` (for example runtimed, which backs the Zed
REPL) cannot deserialize this shape: it is not a plain string, and the
object variant requires `version`. The result was a deserialization
failure that broke the Zed REPL when talking to Deno.
This changes the kernel to report the bare string `"typescript"`, which
is the simplest spec-valid form and is accepted by every consumer. The
maintainer of runtimed asked us to fix it on the Deno side rather than
loosen the protocol type, see
https://github.com/runtimed/runtimed/pull/311#issuecomment-5046801084.
I checked the remaining `language_info` fields (`name`, `version`,
`mimetype`, `file_extension`, `pygments_lexer`, `nbconvert_exporter`)
while I was here; they are all plain strings and were already correct.
`codemirror_mode` was the only offending field.
The existing `jupyter_kernel_info` integration test now also asserts on
`codemirror_mode`.
