[feat]: improve email validation in backend verifier to prevent malform…

[Denisha227]

Overview
Previously, the backend email verifier used a very basic check (if not email or "@" not in email:) to check for valid emails. This allowed invalid or malformed email patterns (such as missing a top-level domain, local-part, or containing invalid characters) to pass through to the subsequent SMTP verification steps, wasting connection bandwidth and server resources.

This PR improves the email verification process by introducing a lightweight, robust regular expression check before SMTP verification.

Key Changes
Regex-Based Check: Added a structured regex pattern (^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+.[a-zA-Z]{2,}$) in contact-discovery-service/verifier.py.
Early Rejection: Ensures malformed email formats (e.g., user@, user@domain, @domain.com, user@domain.) are instantly marked as "invalid" and rejected without triggering unnecessary rate limit checks or DNS/SMTP lookups.
Maintainability: Kept the regular expression lightweight to balance performance and coverage.

[vercel]

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