Ensure all processed strings are null-terminated for safety

[ionux]

Three changes:

1. ok_json.c: add upper-bound guard to the key_len counting loop in okj_find_value_index(). Previously the loop scanned until '\0' with no limit, so a caller passing a non-null-terminated key buffer would cause an out-of-bounds read. The loop now stops at OKJ_MAX_STRING_LEN + 1, which is one byte beyond the longest value any stored token can hold, so lookup behaviour is identical for valid inputs while overlong keys are safely rejected (returned length can never match a stored token).

2. ok_json.h / ok_json.c: add okj_copy_string() — a safe helper that copies an OkJsonString value into a caller-supplied buffer and always writes a null terminator. Copies at most buf_size-1 content bytes, then appends '\0'. Returns the number of characters copied, or 0 on bad arguments (NULL pointers, buf_size == 0). This fills the gap where callers previously had to perform the copy-and-terminate dance themselves, with no library-provided safety net.

3. test/ok_json_tests.c: add six new tests covering the above:

   • test_copy_string_basic — happy-path copy + content check
   • test_copy_string_null_terminated — explicit null-terminator check
   • test_copy_string_truncation — buffer smaller than string
   • test_copy_string_exact_fit — buffer exactly length+1 bytes
   • test_copy_string_null_inputs — NULL str, NULL buf, buf_size==0
   • test_find_key_over_max_len — key > OKJ_MAX_STRING_LEN returns NULL

All 56 tests pass with -Wall -Wextra -Werror -pedantic.

https://claude.ai/code/session_01RYgCNpDtJiR6nMTBXtGbgR