reader::get_execution_price_preview has zero test coverage

[abayomicornelius]

Problem

reader::get_execution_price_preview (contracts/reader/src/lib.rs:594-624) has zero test coverage anywhere in the repository:

grep -rn "get_execution_price_preview" --include="*.rs" . | grep -v reader/src/lib.rs
# no output

It composes two other library calls — get_position_price_impact and get_execution_price — to preview the execution price a trader would receive for a given size/direction before submitting an order. Neither the price-impact computation nor the final execution-price composition is verified through this entry point by any test.

Why it matters

This is explicitly a UI-facing slippage preview ("Useful for the UI to preview slippage before placing an order," per its own doc comment). If the preview diverges from what order_handler::execute_order actually charges (e.g. because one uses mid_price() and the other doesn't, or a future change to get_position_price_impact's signature isn't mirrored here), users would be shown a systematically wrong price with no test to catch the drift.

Suggested fix

Add a test that computes get_execution_price_preview for a known market/size/direction and cross-checks the result against the actual price impact/execution price a real MarketIncrease/MarketDecrease order pays in order_handler's own tests for equivalent inputs.