[7.4] Boundary proposal test — v3 → v4 dispatch + slashing record

Plan reference: plan/glamsterdam-2026-09-05/issues/07-phase-7-devnet-bring-up.md · Issue 7.4

Description

A proposal at the first Gloas slot goes through produce_block_v4 and is recorded in the slashing DB, while the pre-fork slot still uses the v3 path.

Implementation Notes

• Convert MockBlockBeacon to a call-recording mock (Mutex<Vec<Call>> + a canned ProduceBlockResponse) and add the produce_block_v4 arm from 6.2; keep the existing Err("mock") behaviour available so current tests do not change assertions.
• Dispatch is asserted on the BeaconBlockClient seam, not wiremock — production never reaches MockServer. Reuse create_test_config / create_mock_validator_store from coordinator/tests/mod.rs (81, 241), not block_proposal/tests.rs (it has no such helpers).
• Assert the block signing root reaches the signer as DutyRef::BlockRoot { root, slot } (island output), not a tree_hash bridge.
• KAT gate: no test here may be named *_root/*signing_root/*tree_hash — name it test_gloas_slot_dispatches_produce_block_v4 etc. EXEMPTIONS must not grow.

Acceptance Criteria

• RED first: the v4 assertion fails before 6.2's trait arm is wired in
• Slot 2208 records exactly one produce_block_v3 call and zero v4 calls; slot 2240 the inverse, with a BuilderConfig present in the recorded v4 call
• No blinded-block publish is issued for the Gloas slot (publish_blinded_block call count 0)
• D20: with the mock returning BlockContents for slot 2240, exactly one publish_execution_payload_envelope call follows the block publish, carrying the same response's envelope, blobs and proofs, signed under DOMAIN_BEACON_BUILDER; with a bare-block (builder-win) response the call count is 0 and Eth-Builder-Url is echoed
• Slashing DB holds exactly one block record for slot 2240; a replay of the slot is refused
• cargo nextest run -p rvc green — no pre-existing proposal test edited

Testing Notes

call-recording mock for endpoint selection; in-memory slashing DB.

Points: 5 (reason: MockBlockBeacon (coordinator/tests/mod.rs 132) is a unit struct whose produce_block_v3 always returns Err; recording calls and returning a real ProduceBlockResponse reworks create_mock_block_beacon() (172), used at 40 call sites across 11 test files — keep its signature so none of them are edited) · Type: feature · Priority: P0 · Scope: 2 days · Blocked by: #317, P6 (#295 trait ripple, #299 v4 flow, #302 block root, #313 / #315 self-build envelope, D20) · Blocks: #322, #327

Start with coordinator/tests/mod.rs, especially MockBlockBeacon, create_mock_block_beacon(), and the referenced test helpers, then inspect the BeaconBlockClient seam and the existing proposal tests. Run cargo nextest run -p rvc; done means the v3/v4 boundary, envelope, slashing-record and replay assertions pass without editing pre-existing proposal tests.