fix: validate preference value for newsletter

[AmanGIT07]

Currently the value for newsletter traits are not being validated. This PR fixes that.

Request:

{
    "bodies": [
        {
            "name": "newsletter",
            "value": "test" // invalid value
        }
    ]
}

Response:

Status code - 400

Body -

{
    "code": 3,
    "message": "invalid value for preference",
    "details": []
}

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
frontier	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Mar 24, 2025 6:43am

[coveralls]

Pull Request Test Coverage Report for Build 14028912021

Details

• 3 of 36 (8.33%) changed or added relevant lines in 4 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.03%) to 33.382%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
internal/api/v1beta1/preferences.go	3	5	60.0%
core/preference/service.go	0	7	0.0%
core/preference/preference.go	0	8	0.0%
core/preference/validator.go	0	16	0.0%

Files with Coverage Reduction	New Missed Lines	%
core/preference/service.go	1	0.0%

Totals
Change from base Build 14028615747:	-0.03%
Covered Lines:	9143
Relevant Lines:	27389

---

💛 - Coveralls