fix(client-ky): resolve TypeScript type errors
Fix TypeScript compilation errors without using `as any`:
- Use inline array type instead of accessing `KyOptions['retry']['methods']`
to avoid type error when retry can be number | object
- Add type assertion for headers as HeadersInit to satisfy Request constructor
- Change body type from `BodyInit | null` to `unknown` for consistency with
client-fetch and to allow testing with edge case values
- Use `Partial<KyInstance> as KyInstance` pattern in tests (same as client-axios)
to properly type mock ky instances without `as any`