Order::Bundle vs Order::BundleV2 not disambiguated by untagged JSON deserialization

[0w3n-d]

Status

Three tests are currently broken on develop (discovered while re-enabling CI's unit-test job): block_merging::tests::order_json_round_trip_bundle_v2, block_merging::tests::order_json_untagged_disambiguates_bundle_from_bundle_v2, block_merging::tests::block_merging_data_json_round_trip_mixed_orders (all in crates/types/src/block_merging.rs).

Why

Order is a #[serde(untagged)] enum with variants Tx, Bundle, BundleV2. BundleOrder and BundleOrderV2 have overlapping/compatible field shapes (BundleOrderV2 added latest_only as a required field where BundleOrder has it as #[serde(default)]), so untagged deserialization doesn't reliably distinguish them — a BundleV2 payload can deserialize back as a plain Bundle, silently losing which variant was intended.

Affected surface

• crates/types/src/block_merging.rs — Order, BundleOrder, BundleOrderV2, and the 3 tests, currently marked #[ignore] pointing here

Steps

• Decide the disambiguation strategy (e.g. an explicit tag field, or a distinguishing required field on BundleOrderV2 that BundleOrder structurally cannot satisfy)
• Fix and un-ignore the 3 tests