Multi-asset-bounties e2e tests.

[dhirajs0]

Adds an end-to-end test suite for pallet-multi-asset-bounties
on Kusama Asset Hub and Polkadot Asset Hub. Covers the full bounty lifecycle for native KSM, USDT,
and DOT-as-foreign-asset, plus a governance path (SmallSpender referendum)
for fund_bounty. The same shared test tree is wired up on Polkadot Asset
Hub for when the pallet ships there.

What's added

Group	Tests
Native	fund / check status / accept curator / award / full lifecycle
USDT	fund + full lifecycle (with assetRate setup so BalanceConverter admits the spend)
DOT (foreign)	full lifecycle + caller-supplied-values variant
Governance	SmallSpender referendum submitting fund_bounty, decision deposit, force-dispatch under that origin

Shared helpers: ensureUSDTBountySetup / ensureDOTBountySetup (asset rate,
foreign-asset create, treasury seed), V5 asset-kind + beneficiary builders,
preimage helper.

How to verify

yarn test:kusama assetHubKusama.multiAssetBounties             # all MAB tests
yarn test:kusama -t "DOT bounty lifecycle with custom call"    # custom-values
yarn test:kusama -t "SmallSpender referendum for fund_bounty"  # governance

Closes: #598