Deep-clone json and context in init hooks (match searchParams)

[greymoth-jp]

cloneInitHookOptions deep-clones searchParams (#861) and retry so init-hook mutations don't leak across requests, but json and context are still only shallow-cloned (one level). A nested mutation in a beforeRequest hook (e.g. options.json.meta.requestId = ... on a reused json: {meta: {}}) leaks into subsequent requests. Existing tests only cover top-level mutation, so it's undetected.

#861 established the deep-clone-nested-init-hook-structures pattern for searchParams; json/context were left behind. The fix applies the same isolation (+2 tests). Verified both ways: old leaks the nested mutation, fix isolates it; top-level, Date, and class-instance cases preserved, 0 regression.

[sindresorhus]

This regresses cyclic json values when stringifyJson is used and any init hook is present. json is typed as unknown, and stringifyJson is the documented escape hatch for custom serializaton, so Ky should not recursively walk the value before handing it to the custom serializer.

I verified this manually: the same snippet succeeds on origin/main, but this branch throws RangeError: Maximum call stack size exceeded before stringifyJson runs.

I think the underlying leak is real, but deep-cloning arbitrary json is broader than needed. The lower-risk fix is probably to keep json shallow, or add cycle tracking if we really want recursive cloning there. Deep-cloning context makes more sense because it is explicitly a request metadata object, while json can be any serializer input.

[greymoth-jp]

You're right, deep-cloning json is overreach. It's unknown and stringifyJson is the documented escape hatch, so walking it recursively is the wrong default, and with no cycle tracking a cyclic or very deep value overflowed the stack before stringifyJson ran.

Reverted json to a shallow clone (restoring main's behavior); context keeps the deep clone since that's what the leak fix was about. Dropped the nested-json test, kept the nested-context one, and added a regression test for your exact case (cyclic json + custom stringifyJson + an init hook present). Full hooks suite is green.