Refactor foreach loops to explicit LINQ and fix resource disposal

[Copilot]

Addresses code review feedback from PR #8 by refactoring implicit filtering/mapping patterns to explicit LINQ methods and fixing unmanaged resource disposal.

Changes

EntitiesController.cs - Refactored 6 foreach loops:

• Replaced implicit filtering with explicit .Where() predicates
• Replaced immediate variable mapping with explicit .Select() projections
• Materialized queries with .ToList() to prevent deferred execution re-evaluation

UniversesControllerTests.cs - Fixed resource leaks:

• Added using var declarations for FormUrlEncodedContent instances

Example

Before:

foreach (var line in aliasLines)
{
    var trimmed = line.Trim();
    if (!string.IsNullOrWhiteSpace(trimmed))
    {
        newAliases.Add(trimmed);
    }
}

After:

var validAliases = aliasLines
    .Select(line => line.Trim())
    .Where(trimmed => !string.IsNullOrWhiteSpace(trimmed))
    .ToList();

foreach (var trimmed in validAliases)
{
    newAliases.Add(trimmed);
}

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