[0x2me]

Description

Finishes #2507. Error::ProtocolFeeOnStaticOrder was already removed in #4832; this is the dependent cleanup the issue asks for: fee on Fulfillment and JitTrade in solvers-dto becomes a plain U256 instead of Option<U256>.

Same approach as #3822 (approved in principle there), but narrower: the driver surplus_fee() refactor is left for a follow-up so this stays a small review.

Breaking change to the solver API (crates/solvers/openapi.yml). fee on Fulfillment was documented as "only gets returned for limit orders" and is now required. A solver response containing a fulfillment without fee is rejected at deserialisation by the driver, including on the quoting path, and the whole response is discarded, not just that trade. JitTrade already listed fee as required, so this only tightens the Fulfillment half. Every in-tree solver always emitted fee, so the exposure is limited to external solver engines that relied on the old wording. If a softer landing is preferred, #[serde(default)] on the two fields delivers the same code cleanup while keeping old payloads working; happy to switch.

Changes

• solvers-dto: fee: U256 on Fulfillment and JitTrade; serde tests for missing and present fee.
• solvers: drop the Some(..) wrapping in the solution DTO.
• driver: read fee directly instead of unwrap_or_default(); the test mock solver always emits fee.
• e2e: fixtures updated.
• openapi.yml: fee added to Fulfillment.required, description rewritten and typed as TokenAmount.

How to test

1. cargo nextest run -p solvers-dto (new: fulfillment_requires_fee, fulfillment_fee_round_trips, jit_trade_requires_fee).
2. cargo nextest run and cargo clippy --locked --workspace --all-features --all-targets -- -D warnings: green locally.
3. RUST_MIN_STACK=3145728 cargo nextest run -p driver --test-threads 1 --run-ignored ignored-only: 96/96 locally.
4. E2E suite was compile-checked only (cargo check -p e2e --tests), not run locally; please watch CI.

Related Issues

Fixes #2507

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recheck

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This pull request has been marked as stale because it has been inactive a while. Please update this pull request or it will be automatically closed.

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Reopened as #5100 (rebased on main; GitHub would not reopen this one after the force-push).