fix: prevent duplicate lead creation by mobile number

[debgotwired]

Summary

• prevent creating duplicate active leads with the same mobile number across formatting variants
• keep self-updates allowed and ignore converted leads
• add lead integration coverage for duplicate mobile validation

Testing

• python3 -m py_compile crm/fcrm/doctype/crm_lead/crm_lead.py crm/fcrm/doctype/crm_lead/test_crm_lead.py
• git diff --check
• bench --site crmtest.local run-tests --module crm.fcrm.doctype.crm_lead.test_crm_lead