Refactor TreeEnsemble weight loop: validate array sizes once before iteration

[Copilot]

Description

Addresses review feedback on #27677. The three per-iteration ORT_ENFORCE bounds checks inside the weight assignment loop in tree_ensemble_common.h have been moved to a single pre-loop validation block.

Before: on every iteration, the loop checked i < target_class_ids.size(), i < target_class_weights.size(), and i < target_class_weights_as_tensor.size().

After: since indices is built from target_class_nodeids with consecutive indices [0, limit), i = indices[indi].second is always in range — so size relationships are validated once upfront with == limit equality checks:

limit = attributes.target_class_nodeids.size();
ORT_ENFORCE(attributes.target_class_ids.size() == limit,
            "target_class_ids size (", attributes.target_class_ids.size(),
            ") must match target_class_nodeids size (", limit, ")");
ORT_ENFORCE(attributes.target_class_weights_as_tensor.empty() ||
                attributes.target_class_weights_as_tensor.size() == limit, ...);
ORT_ENFORCE(!attributes.target_class_weights_as_tensor.empty() ||
                attributes.target_class_weights.size() == limit, ...);
for (indi = 0; indi < limit; ++indi) {
    // no per-element bounds checks needed
}

Motivation and Context

The per-iteration checks made the control flow harder to reason about and repeated redundant work on every loop iteration. Validating the authoritative array lengths once — before the loop — makes it clear which arrays must be parallel and eliminates repeated checks.

Original PR: #27677

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