## Summary
- Fix `TagPriorityName()` to strip comma-separated tag options (e.g.,
`omitempty`) from tag values
- Before: `json:"user_name,omitempty"` → field name =
`user_name,omitempty`
- After: `json:"user_name,omitempty"` → field name = `user_name`
- Aligns with `structcache.genPriorityTagAndFieldName()` which already
handles this correctly
- When tag name is empty (e.g., `gconv:",omitempty"`), continues to next
priority tag instead of breaking
## Test plan
- [x] Reproduced bug: `RuleFuncInput.Field` was `user_name,omitempty`
instead of `user_name`
- [x] Verified fix: field name correctly extracted as `user_name`
- [x] Verified fallthrough: `gconv:",omitempty"` + `json:"name"` → uses
`name`
- [x] Existing `Test_Fields_TagPriorityName` passes
- [x] Full `os/gstructs` test suite passes
- [x] Full `util/gvalid` test suite passes
- [x] Full `util/gconv` test suite passes
closes#4665