[SalesForce] Lead Status Converted Value to Contact Status

Workflows, Validation Rules and Before triggers fire on Lead Conversion only if the Setup > Customise > Leads > Settings > Enable validations and triggers is checked.

It might be worth checking if this setting is enabled, because otherwise the Lead Status value will not be synced to your custom field on Lead Conversion via workflow, and therefore not be copied across.

(Even then, I believe the Lead Triggers / Workflows fire after the Account and Contact ones, so it may be to late for the Lead Status to copy across)

The order of execution at Lead Conversion has been answered here

This can be achieved by a trivial LeadAfter trigger, or if the Lead Status is always set to Qualified upon Conversion, then you could set 'Qualified' as the Default Value on the Contact Lead Status field.

(One way of doing it also may be to only allow 'Qualified' Leads to be Converted - this can be controlled by having a different layout for Qualified Leads, when the status changes to Qualified, a workflow fires - changes RecordType to QualifiedLead and therefore associated Page Layout changes, which now has the Convert Button. With this approach, the Lead Status at conversion is available in advance and available to be copied across at Lead Conversion)

Your problem is in this piece of code:

List<Contact> leads = [
    SELECT 
        Id, Email 
    FROM 
        Contact
    WHERE 
        Email IN :leadEmails
];

for(Lead singleLead:leads){
    singleLead.Status = 'Converted';
}

Notice how yourleads variable is a List<Contact>. You then try to loop over that list and cast each item in the List as a Lead. You can not do this. Those items are Contacts.

Try something like this out instead:

trigger ConvertLead on Lead (before insert) {
    List<String> leadEmails = new List<String>();
    for(Lead lead:Trigger.new){
        leadEmails.add(lead.Email);
    }

    List<Contact> contacts = [
        SELECT 
            Id, Email 
        FROM 
            Contact
        WHERE 
            Email IN :leadEmails
    ];

    Set<String> contactEmails = new Set<String>();
    for(Contact contact:contacts){
        contactEmails.add(contact.Email);
    }

    for(Lead lead:Trigger.new){
        if(contactEmails.contains(lead.Email){
            lead.Status = 'Converted';
        }
    }
}

First off, notice I changed the trigger to a before insert. The reason for this is because you will receive an error if you attempt to perform a DML operation on the object a trigger is run on in an after context. The reason this is not allowed is because it would put the code in an infinite loop.

Secondly, notice that I changed your variable names to be more descriptive. I renamed your Contact list variable to contacts. This is a core concept of Clean Code.

Thirdly, I used a Set to store all matching Contact emails in the database. This will only keep one value for each of the unique email addresses found by the SOQL query.

Finally, you will notice I removed all DML operations. With the trigger changing to before insert, there is no need to run a DML operation as the object will be inserted directly after the trigger runs.

NOTE: This is not production ready code. I have not attempted to run it and wrote it specifically on this site. You must write unit tests for it and do your own testing to verify it does what you intend. Good luck!