fix: support time_format tag in JSON binding (ShouldBindJSON)

[waterWang]

Description

The time_format struct tag works for form/query binding but is completely ignored when binding via JSON (ShouldBindJSON). This has been a long-standing issue open since 2019 (see #2170).

Root cause: The JSON binding path (binding/json.go) uses json.Decoder.Decode(obj) directly, which calls time.Time.UnmarshalJSON — this only supports RFC3339 format. The time_format/time_utc/time_location tags are only checked in the form binding path (form_mapping.go → setTimeField).

Fix: When a struct has time.Time fields with time_format tags, the decoder falls back to mappingByPtr with a jsonSource setter that uses the existing setTimeField function. For structs without time_format tags, the original fast path (direct JSON unmarshal with UseNumber/DisallowUnknownFields support) is preserved.

Changes

• binding/json.go: Added jsonSource type that implements setter interface. For time.Time fields with time_format tags, it uses setTimeField to parse the string value with the custom format. For all other types, it uses standard JSON unmarshaling. Modified decodeJSON to check for time_format tags and fall back to the jsonSource approach when needed.
• binding/json_test.go: Added comprehensive tests covering:
  • Custom time format strings (e.g. "2006-01-02 15:04:05")
  • Unix timestamp formats (unix, unixNano)
  • time_utc tag combination
  • time_location tag combination
  • Regression: RFC3339 format still works without time_format tag

Backward Compatibility

✅ 100% backward compatible — all existing tests pass unchanged. The fast path (direct JSON unmarshal) is used for structs without time_format tags, so UseNumber and DisallowUnknownFields settings are unaffected.

Fixes #2170