Refactor SettingsBase cache lookup to use explicit LINQ filtering

[Copilot]

Addresses feedback on #1147 to replace implicit filtering within a foreach loop with explicit LINQ filtering.

Changes

• Replaced foreach with FirstOrDefault: Changed implicit if (kvp.Value != null) check inside loop to explicit .FirstOrDefault(kvp => kvp.Value != null) predicate
• Safer null handling: Check firstPair.Key instead of firstPair.Value to distinguish actual dictionary entries from default KeyValuePair struct
• Added System.Linq using: Required for LINQ extension methods

Before/After

// Before: implicit filtering
foreach (var kvp in AkavacheBuilder.BlobCaches)
{
    if (kvp.Value != null)
    {
        return kvp.Value;
    }
}

// After: explicit filtering
var firstPair = AkavacheBuilder.BlobCaches
    .FirstOrDefault(kvp => kvp.Value != null);

if (!string.IsNullOrEmpty(firstPair.Key))
{
    return firstPair.Value!;
}

The fallback chain to CacheDatabase caches remains unchanged when no cache is found in AkavacheBuilder.BlobCaches.

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[github-actions]

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