test: add sell protocol fee recipient balance regression

[wheval]

Summary

Adds a regression test that verifies the protocol fee recipient balance increases by the correct amount after a sell, and wires sell execution to accrue that balance.

Purpose / Motivation

Sell event and quote tests confirm fee fields are computed, but they do not prove the protocol fee recipient actually receives the fee. A direct balance assertion closes that gap and guards against regressions where fees are calculated but not credited.

Changes Made

• Add regression test for protocol fee recipient receiving correct amount on sell #296: Add regression test for protocol fee recipient receiving correct amount on sell
• Credit the configured protocol fee recipient balance during sell_key using the current fee config and key price
• Add get_protocol_recipient_balance read method for test and indexer visibility
• New integration tests assert balance before/after sell matches the bps-derived protocol fee and the sell quote

How to Test

1. From the repo root, run cargo test --test sell_protocol_fee_recipient_balance.
2. Confirm both tests pass:
   • test_sell_increases_protocol_fee_recipient_balance_by_bps_fee
   • test_sell_protocol_fee_recipient_balance_accumulates_across_two_sells
3. Run cargo test --workspace to confirm no regressions in existing sell and fee tests.

Expected: protocol fee recipient balance is 0 before the first sell and increases by floor(price * protocol_bps / 10_000) after each sell.

Breaking Changes

• None for callers that do not set a protocol fee recipient.
• When a protocol fee recipient, key price, and fee config are all configured, sell_key now accrues the protocol fee share to get_protocol_recipient_balance.

Related Issues

Closes #296

Checklist

• Code builds successfully
• Tests added/updated
• No console errors
• Documentation updated (if needed)

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