Fix LDAP bind panic when userPassword attribute is missing

[longsleep]

Summary

Fixes #114 - Resolves LDAP bind panic and silent error handling when user records lack userPassword attributes.

This PR addresses two critical issues:

1. Silent panic recovery - panics are now properly logged with context
2. Unsafe array access - prevents panic when accessing missing userPassword values

Changes Made

Core Fix

• pkg/ldapserver/bind.go: Enhanced panic logging with error context and remote address
• server/handler/ldif/entry.go: Added safe password validation with proper null checks
• server/handler/ldif/entry_test.go: Comprehensive unit tests covering edge cases

Key Improvements

• ✅ No panics when processing entries without userPassword
• ✅ Clear, actionable error messages in logs ("user has no password attribute")
• ✅ Proper LDAP error codes (InvalidCredentials vs OperationsError)
• ✅ No timing attack vulnerabilities (maintains constant-time behavior)
• ✅ Backward compatibility maintained

Testing

Unit Tests

• Added comprehensive test suite for password validation edge cases
• Tests for missing userPassword attribute
• Tests for empty password values
• Performance benchmarks to ensure no regression

End-to-End Testing

• Verified no panics with malformed LDIF data containing users without passwords
• Confirmed proper error codes (InvalidCredentials 49) returned to clients
• Validated clear debug logging for troubleshooting
• Ensured existing functionality unchanged

Before/After Behavior

Before (Issue #114):

• Panic when user has no userPassword → silent recovery → OperationsError
• No logging of panic reason → difficult debugging

After (This Fix):

• Safe handling of missing userPassword → InvalidCredentials
• Clear log: "ldap bind credentials error" error="user has no password attribute"
• Proper error codes for client applications

Security Considerations

• No timing attack vulnerabilities introduced
• Maintains existing authentication security model
• No sensitive information leaked in logs

Backward Compatibility

• No changes to public APIs
• Existing LDIF files continue to work
• Error response codes consistent for valid scenarios

Test Plan

• All existing tests pass
• New unit tests cover edge cases
• Manual testing with malformed LDIF data
• Performance regression testing
• Error code validation

Ready for review and merge.