feat(settlement): return semantic errors for batch validation failures

[datagerl]

Summary
replace generic batch validation panics with typed settlement contract errors
add focused tests for empty, oversized, and non-positive batches
update settlement error docs and error-code coverage
Changes
introduce BatchEmpty and BatchTooLarge settlement errors
return AmountNotPositive for non-positive batch amounts
add regression tests for invalid batch inputs
document the new visible error behavior
Testing
verified editor diagnostics for the touched Rust files note: full cargo test could not be executed in this environment because the Rust toolchain was unavailable on PATH

Closes #732

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