[WIP] ORT aborts on ConcatFromSequence with empty sequence inputs

[Copilot]

• Analyzed the issue: ConcatFromSequence aborts when mixing empty and non-empty tensors with different ranks
• Identified the root cause: validation loop in PrepareForCompute() only checks tensors after the reference tensor, missing validation for tensors before the reference
• Create test case to reproduce the specific issue (mixing empty and non-empty tensors with different ranks)
• Fix the validation logic to check ALL tensors against the reference tensor
• Verify the fix works and doesn't break existing functionality
• Run existing tests to ensure no regressions

Root cause: In concat.cc PrepareForCompute(), when finding the first non-empty tensor as reference, the validation loop starts from reference_tensor_index + 1, which skips validating empty tensors that appear before the reference tensor. This allows incompatible tensor ranks to pass validation, causing aborts later during computation.

Fixes #16619.

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