⚙️ Core: Refactor AllocationStrategy to use ClinicalTrialError

[fderuiter]

🔧 The Violation: AllocationStrategy used String for error handling (Primitive Obsession), preventing programmatic handling of errors like InsufficientSampleSize or InvalidData.
🛠️ The Fix: Enforced Typed Errors. Updated AllocationStrategy to return Result<Vec<Group>, ClinicalTrialError>. Updated SimpleRandomizer and BlockRandomizer to match.
📉 Debt Reduction: Removed ad-hoc string error construction in the core strategy.
🧪 Integrity: Verified with cargo test and cargo clippy. Logic preserved, backward compatibility maintained via legacy adapter functions.

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PR created automatically by Jules for task 941870691016806650 started by @fderuiter

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