chore(fee-processor): add regression tests and improve overall coverage

[F3Joule]

Overview

This PR improves test coverage for pallet-fee-processor and pallet-referrals following the fee processing refactor. It also aligns the test mock with the production HdxFeeReceivers configuration.

Found issues / follow-up

• ⚠️ process_trade_fee(amount=0) inserts asset into PendingConversions despite no tokens being transferred, leading to infinite failed conversions in on_idle. Needs a decision: early return on zero amount, or guard in do_convert.
• ⚠️ No governance extrinsic exists to force-remove an asset from PendingConversions. A permanently failing asset (e.g. no liquidity pool) will consume an on_idle slot every block indefinitely.

Changes

pallet-fee-processor

Mock updated to reflect production HDX receiver shape
Replaced the previous 2-receiver mock (50/50 including a fake referrals receiver) with 3 receivers matching the structural shape of the runtime config:

• HdxGigaHdxFeeReceiver — 70%
• HdxGigaRewardFeeReceiver — 20%
• HdxStakingFeeReceiver — 10%

Referrals is not part of the HDX fee path and has been removed from HdxFeeReceivers in the mock accordingly.

New tests — convert.rs

• on_idle_retries_failed_conversion_on_next_block - confirms retry behavior (intentional divergence from old referrals pallet which removed failed assets)
• convert_extrinsic_for_asset_not_in_pending_still_executes - documents permissionless nature of do_convert
• on_idle_returns_zero_when_weight_below_single_conversion - verifies weight guard early exit
• on_idle_processes_only_one_when_weight_fits_exactly_one - verifies weight budget precision
• distribute_to_pots_uses_total_param_not_actual_pot_balance - regression test for suspicious unused _balance read at lib.rs:280; confirms pre-existing HDX on pot is not redistributed

New tests — process_fee.rs

• process_trade_fee_same_asset_twice_does_not_duplicate_pending_count
• process_trade_fee_fails_when_source_has_insufficient_balance
• process_trade_fee_nothing_changes_when_pre_deposit_callback_fails - confirms Substrate transactional rollback, so we are sure no assets stuck on pot because of failing handler.

pallet-referrals

New tests — accumulator.rs (new file)

Covers the new reward-per-share accumulator logic that replaced the old share-based distribution:

• on_hdx_deposited_bumps_reward_per_share_correctly
• on_hdx_deposited_does_nothing_when_total_shares_is_zero
• on_hdx_deposited_accumulates_correctly_over_multiple_calls
• on_fee_received_calculates_shares_correctly_for_all_parties
• on_fee_received_skips_referrer_share_when_no_code_linked
• on_fee_received_external_account_none_shares_dropped
• checkpoint_user_accumulates_pending_rewards_before_share_change
• claim_rewards_cannot_double_claim_via_debt_mechanism
• claim_rewards_combines_accumulated_and_pending_rewards

ExtBuilder extended with with_reward_per_share, with_user_reward_debts, with_user_accumulated_rewards builders to support accumulator state setup in tests.

[github-actions]

Crate versions that have not been updated:

• pallet-fee-processor: v0.1.0
• pallet-referrals: v1.5.0

Runtime version has not been increased.