Unexport LdapImpl.Config and expose via getter method

[Copilot]

LdapImpl.Config was exported, unintentionally widening the public API and allowing direct struct mutation. This replaces the exported field with an unexported config and a GetConfig() *ConfigLDAP getter.

Changes

• ldap.go: Config *ConfigLDAP → config *ConfigLDAP with GetConfig() *ConfigLDAP getter; all internal references updated
• cmd/ldapfetch/main.go: Replaced struct literal initialization with NewAuthenticator_LDAP(ldapConf) constructor

// Before
ldapImpl := authaus.LdapImpl{Config: ldapConf}

// After
ldapImpl := authaus.NewAuthenticator_LDAP(ldapConf)

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