Fix LDAP injection level 1 filter escaping

[r1kka6]

Summary

• escape the level 1 username before embedding it in the LDAP filter
• keep the change limited to the level 1 query construction in LDAPInjectionVulnerability.java

Verification

• before the fix, GET /VulnerableApp/LDAPInjectionVulnerability/LEVEL_1?username=* returned all LDAP users via (uid=*)
• after the fix, the same username=* request and username=*)(uid=* both return No users found
• a normal username=alice request still returns only alice