Snapshot AddFrom's items so a collection can add from itself [patch]

[matt-edmondson]

Fixes #167

Problem

CollectionExtensions.AddFrom enumerated items while adding to collection. When items was the collection itself, or a lazy query over it, the first Add invalidated the enumerator:

List<int> l = [1, 2];
l.AddFrom(l);                    // InvalidOperationException: Collection was modified
l.AddFrom(l.Where(x => x > 0));  // same

List<T>.AddRange(l) handles the same call and produces [1, 2, 1, 2].

Change

AddFrom now materialises items into an array before adding any of them. That is the same snapshot ReplaceWith already takes for the same aliasing case (#132). The snapshot is unconditional, because a lazy query over the target can't be detected from its type.

Tests

• AddFromItselfDoublesTheCollection: l.AddFrom(l) gives [1, 2, 1, 2].
• AddFromALazyQueryOverItselfAddsTheQueriedItems: l.AddFrom(l.Where(x => x > 0)) gives [1, 2, 1, 2].

With the fix reverted, both tests fail. With it in place, the full suite passes: 200 tests, 0 failed.

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Generated by Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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