Abhinav Raut 495ff02067 fix[sla]: incorrect calculated deadline when SLA minutes ends exact at a working day's closing time. ...

Example test case for it.
```go
		{
			name:      "Monday to Friday 10:00 to 18:00",
			startTime: time.Date(2025, 03, 22, 18, 1, 43, 0, locIST), // Sat
			// 24 hours.
			slaMinutes: 1440,
			businessHours: models.BusinessHours{
				Hours: mustMarshalJSON(map[string]models.WorkingHours{
					"Monday":    {Open: "10:00", Close: "18:00"},
					"Tuesday":   {Open: "10:00", Close: "18:00"},
					"Wednesday": {Open: "10:00", Close: "18:00"},
					"Thursday":  {Open: "10:00", Close: "18:00"},
					"Friday":    {Open: "10:00", Close: "18:00"},
					"Saturday":  {Open: "10:00", Close: "14:00"},
				}),
			},
			timeZone:       "Asia/Kolkata",
			expectedResult: time.Date(2025, 03, 26, 18, 0, 0, 0, locIST),
		},
```
fix: more validations for working hours.
chore: remove unused struct fields in working hours

2025-03-26 23:56:13 +05:30

..

models.go

fix[sla]: incorrect calculated deadline when SLA minutes ends exact at a working day's closing time.

2025-03-26 23:56:13 +05:30