Fix Sequence Validation Code Truncation

[Siddharthakhandelwal]

The Pydantic request model for sequence validation (validate_amino_acids) is incomplete and currently breaks mid-line. Without input validation, bad protein sequences can crash the upstream model.
Tasks/Acceptance Criteria:

• Complete the validate_amino_acids Pydantic logic rejecting non-whitelisted characters or invalid lengths.
• Ensure the backend returns consistent HTTP 422 or 400 error codes when validation fails.
• Format the error response safely as a clear JSON payload indicating the exact problem to the user.
• Catch and gracefully render these validation errors in the frontend via a toast notification.