[BREAKING] Make solver trade fee non-optional

[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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Supersedes #4964, which the stale bot closed before review and GitHub would not reopen after the rebase. Rebased on current main; one conflict in the driver solution DTO resolved (the haircut_fee argument was removed upstream).

@fleupold @MartinquaXD could one of you take a look and approve the CI run when you get a chance? Happy to switch to #[serde(default)] if you prefer the softer landing described above.