fix(ldap): case-insensitive attribute lookup, request login attribute in search

[allamiro]

Fixes #292.

Problem

Two related bugs broke LDAP login against OpenLDAP even with correct credentials:

1. Attribute lookups in the search result were case-sensitive. LDAP attribute names are case-insensitive (RFC 4512 §2.5) and servers return their own canonical casing — OpenLDAP returns cn for a requested CN, so login failed with 'CN attribute not found'.
2. The user search only requested the CN attribute, so the login attribute (e.g. uid) was never in the result and its lookup failed next ('login attribute uid not found').

Changes

• get_attr_ci() helper for case-insensitive attribute lookup, used for CN, login attribute, database/keyfile attributes
• the search now requests the login attribute alongside CN
• unit test for the case-insensitive lookup

Testing

• cargo test: 10/10 pass
• End-to-end against a seeded OpenLDAP 2.6 container (from the compose setup in PR Bump clipboard-polyfill from 2.8.6 to 4.0.1 #4): valid credentials → 200 + session, wrong password → 401 (invalidCredentials from user bind), empty password → rejected

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Summary by cubic

Fix LDAP login failures by using case-insensitive attribute lookup and requesting the login attribute in the search. Restores compatibility with OpenLDAP and other servers that return canonical-cased attributes.

• Bug Fixes
  • Added get_attr_ci() and used it for CN, login, database, and keyfile attributes.
  • Search now requests CN plus the configured login attribute (e.g., uid).
  • Added a unit test for the case-insensitive lookup.

^{Written for commit 8743667. Summary will update on new commits.}