Check every output for duplicate producers

[Evangelink]

Summary

CheckForDuplicateOutputs returns from the entire method after registering the first previously unseen output instead of continuing to the next output.

Impact

A node registers at most its first ordinary output in _outputProducer. Duplicate-path conflicts and invalid reads of another project's outputs are therefore not detected for the remaining outputs. This disables an important cache-correctness diagnostic on both cache-hit and post-build paths.

Evidence

In src/Common/MSBuildCachePluginBase.cs:

foreach (...)
{
    NodeContext previousNode = _outputProducer.GetOrAdd(relativeFilePath, nodeContext);
    if (previousNode == nodeContext)
    {
        return;
    }
    ...
}

The control flow contradicts the per-output loop and comment.

Suggested fix

Continue to the next output after successfully registering a first producer. Add tests with multiple outputs where only a later path is duplicated, covering identical and conflicting content as well as producer ordering.

Acceptance criteria

• Every output is registered and checked.
• A duplicate in any position is detected.
• Allowed identical duplicates retain their current behavior.
• Both cache-hit and completed-build paths are covered.