refactor(lifi): split direct and discount routes

[1kresh]

Aligns the LI.FI executor with the RFQ executor (swapInputs/discountSwapInputs) and the UniswapX executor (routes/discountRoutes): direct-swap legs and discount-backed legs now live in separate arrays instead of one FillRoute array with an embedded optional FillDiscount.

Changes

• ILiquidLaneLifiExecutor: removed FillDiscount (and discountId); FillRoute is now {adapter, amountIn, amountOut} and a new DiscountRoute {adapter, amountIn, discountSwap, protocolSignature} mirrors the UniswapX executor's struct. FillCall and finaliseWithCurrentTimestamp carry both arrays.
• LiquidLaneLifiExecutor.orderFinalised: two loops in the UniswapX executor's style — transfer + direct swap() per route, then transfer + discount swap() per discount route — replacing the discountId == 0 branch.
• Tests updated for the new signature; the discount test now exercises a discount-only fill, and a new testFinaliseWithCurrentTimestampExecutesDirectAndDiscountRoutes covers a mixed fill.

Verification

• 34/34 lifi tests pass, including the Sepolia fork test against the deployed InputSettlerEscrow
• Full suite: 103 passed, 0 failed
• forge fmt --check clean

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