fix: improve JSON tag handling and validation logic to support omitzero

[yockii]

1. to support new tag omitzero for go 1.24+
2. fix some tags in json like json:"id,omitempty,string"
3. fix null filed in json string, when the field is not required, and it is "null" value, the validation may failed.
4. add test for these situations.

eg:

type User struct {
    ID uint64 `json:"id,omitempty,string"`
    Name string `json:"name"`
    Age int `json:"age,omitzero"`
    Addr Address `json:"address,omitempty"`
}

type Address struct {
  Zip string `json:"zip"`
}

and with json:

{
  "name": "Yockii",
  "address": null
}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.59%. Comparing base (8e5611c) to head (ba2ee13).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1076   +/-   ##
=======================================
  Coverage   99.59%   99.59%           
=======================================
  Files          34       34           
  Lines        2206     2216   +10     
=======================================
+ Hits         2197     2207   +10     
  Misses          6        6           
  Partials        3        3           

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