Fix bridges integration test failure for the Westend network

[rosarp]

Description

Fix bridges integration test failure for the Westend network by using the slot-based collator for asset-hub-westend nodes.

The asset-hub-westend runtime configures RelayParentOffset = 1 (at cumulus/parachains/runtimes/assets/asset-hub-westend/src/lib.rs:138) in this commit , which requires relay parent descendant headers in the parachain inherent data. However, the zombienet test was launching the collators with the default look ahead authoring policy, which passes an empty relay_parent_descendants vec. This caused a runtime panic on every block build attempt:

  Unable to verify provided relay parent descendants.
  expected_rp_descendants_num: 1
  error: InvalidNumberOfDescendants { expected: 2, received: 0 }

The parachain remained stuck at block #0, failing the test assertion:
asset-hub-westend-collator1: reports block height is at least 10 within 180 seconds

Only the slot-based collator (collators/slot_based/block_builder_task.rs) calls create_inherent_data_with_rp_offset() with the required descendant data. The look ahead and basic collators call create_inherent_data() which passes None.

Integration

This change only affects a zombienet test TOML configuration file. No crate changes.

Review Notes

The fix adds "--authoring", "slot-based" to both asset-hub-westend-collator1 and asset-hub-westend-collator2 in
bridges/testing/environments/rococo-westend/bridge_hub_westend_local_network.toml.

This is only needed for the Westend side because:

• asset-hub-westend has RelayParentOffset = ConstU32<1> — requires descendant headers
• asset-hub-rococo has RelayParentOffset = ConstU32<0> — no descendant verification, works with any collator

The Rococo TOML (bridge_hub_rococo_local_network.toml) is unchanged since its asset-hub runtime doesn't require relay parent descendants.

[rosarp]

Found while testing paritytech/parity-bridges-common#3170