common: json_to_s64 and json_to_double still accept hex, octal and signs

#9150 routed `json_to_u64()` through the new `str_to_u64()`, which accepts strict decimal only. `json_to_s64()` and `json_to_double()` still use `strtoll`/`strtod` with base 0, so they accept hex (`0x...`), octal, and a leading `+`/`-`, none of which are valid JSON numbers. `u64` is now asymmetric with the other two.

Note the `u64` change already altered one case: `"010"` used to parse as 8 (base-0 octal) and now parses as 10. Correct per JSON, but worth being aware of when tightening the others.

No urgency, this has been the parsing behaviour for a long time.