test(auth): cover SES bounce shapes and the EmailBounce model
Because: - The bounce handler tests covered few SES notification shapes. - The EmailBounce model had no spec for id mapping or error handling. This commit: - Adds a SES notification shapes block to bounces.spec.ts. - Covers bounce and complaint types, diagnosticCode edge cases, and no headers. - Covers two recipients and a createEmailBounce rejection. - Adds email-bounce.spec.ts in fxa-shared for the EmailBounce model. Closes FXA-12838