This updates our generated TypeScript types to be more correct with how
the server actually behaves, **specifically for JSON-RPC requests**.
Before this PR, we'd generate `field: T | null`. After this PR, we will
have `field?: T | null`. The latter matches how the server actually
works, in that if an optional field is omitted, the server will treat it
as null. This also makes it less annoying in theory for clients to
upgrade to newer versions of Codex, since adding a new optional field to
a JSON-RPC request should not require a client change.
NOTE: This only applies to JSON-RPC requests. All other payloads (i.e.
responses, notifications) will return `field: T | null` as usual.
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