Trading rules missing for dynamically registered pairs on per-pair-rules connectors (KeyError, silent zombie executors on XRPL)

[fengtality]

Summary

Sibling of #207, same root cause, different pair-derived state left stale. Trading connectors are created with trading_pairs=[], so connectors that build trading rules per pair (XRPL queries tick size / transfer rate / AMM info on-ledger for each pair in self._trading_pairs) initialize with an empty _trading_rules dict. When a pair is later registered dynamically, add_market() initializes the order book but never adds the pair to connector._trading_pairs nor re-runs _update_trading_rules(). Any executor created for that pair then dies at startup:

2026-08-07 05:33:24,767 - hummingbot.core.utils.async_utils - ERROR - Unhandled error in background task: 'USDC-XRP'
Traceback (most recent call last):
  File ".../hummingbot/core/utils/async_utils.py", line 9, in safe_wrapper
    return await c
  File ".../hummingbot/strategy_v2/executors/executor_base.py", line 194, in control_loop
    await self.on_start()
  File ".../hummingbot/strategy_v2/executors/executor_base.py", line 182, in on_start
    await self.validate_sufficient_balance()
  File ".../hummingbot/strategy_v2/executors/order_executor/order_executor.py", line 388, in validate_sufficient_balance
    price_for_validation = self.get_price_for_balance_validation()
  ...
    trading_rule = self._trading_rules[trading_pair]
KeyError: 'USDC-XRP'

Symptoms

• POST /executors/ returns 201 Created, the executor registers as RUNNING, but its control loop is already dead: order_id: None, current_retries: 0, no order ever reaches the ledger. From the API consumer's perspective the order was placed; on-chain nothing happened. This zombie state is arguably worse than AsyncThrottler missing pair-specific rate limits for dynamically registered trading pairs (AttributeError: 'NoneType' object has no attribute 'weight') #207's visible 500.
• Reproduced live on xrpl / USDC-XRP and RLUSD-XRP (any XRPL pair): 3/3 executor creates crashed identically (2026-08-07, Docker hummingbot/hummingbot-api:latest).

Root cause

• UnifiedConnectorService._create_trading_connector() builds connectors with trading_pairs=[] (services/unified_connector_service.py:695).
• XRPL's _update_trading_rules() iterates only self._trading_pairs (hummingbot/connector/exchange/xrpl/xrpl_exchange.py, _make_trading_rules_request_impl), so no rules are ever built.
• The dynamic registration path — ExecutorService._prepare_market() → TradingService.add_market() → order-book init in UnifiedConnectorService — touches only the order book tracker. Neither connector._trading_pairs nor trading rules are updated. The base ExchangePyBase.add_trading_pair() likewise only subscribes the order book.
• The 30-minute _trading_rules_polling_loop cannot self-heal it either, since it re-reads the same empty _trading_pairs.

Most CEX connectors dodge this because one exchange-info request returns rules for all pairs regardless of _trading_pairs. Any connector building rules from self._trading_pairs (XRPL today; others with per-pair rule construction) is affected.

Relationship to #207

Same defect, different victim: #207 is the AsyncThrottler never learning pair-templated rate limits; this is trading rules never being built. Both stem from dynamic pair registration not re-syncing pair-derived connector state, and #207's suggested add_rate_limits() sync does not fix this one (nor vice versa). Suggests a single registration helper that syncs all pair-derived state — _trading_pairs, throttler limits, trading rules — at every registration site, including the data-connector bootstrap path #207 already flags as bypassing registration helpers.

PR to follow addressing both.