feat(accounts): fast add-credential with immediate balances + background bring-up

[fengtality]

Problem

Adding a connector credential blocks on the full trading-connector bring-up before returning. For Hyperliquid perp that is ~85s — a metaAndAssetCtxs call per HIP-3 DEX in both symbol-map init and trading-rules update, plus order-book / websocket startup waits. The result:

• the UI sits on "Saving credentials…" for up to a minute, and
• the new connector's balances only appear a refresh loop later (and a full update_account_state() over all connectors — including an unreachable Gateway — ran on every add).

Change

Split the trading-connector bring-up into a fast part (balances only) and a heavy part (the rest), and finish the heavy part in the background.

• update_connector_keys now creates the connector, fetches balances only (mirroring the Hummingbot CLI connect), caches it, and schedules the heavy bring-up (symbol map, trading rules, HIP-3 markets, positions, recorders, network) in the background — in place on the cached connector, so it becomes trade-ready without blocking the response or flickering the portfolio. Bad keys still fail fast here and the caller deletes the credential.
• _create_and_initialize_trading_connector keeps the exact same end state by calling the new _finish_trading_connector_init after the fast balances fetch.
• add_credentials surfaces the new balances immediately via a scoped update_account_state(account_names=[…], connector_names=[…], skip_gateway=True) instead of a full all-connectors / Gateway refresh. It's wrapped in its own try/except so a display-only refresh failure can never delete a just-validated credential.

Correctness

• Delete-race guarded: the background task checks list_available_credentials before and after finishing, and tears the connector back down if the credential was removed meanwhile — so a removed key can't linger in the cache / account state / portfolio.
• Background tasks are held in a module-level set so they aren't garbage-collected mid-run.
• No public API or response-shape changes; the connector reaches the identical fully-initialized state, just asynchronously.

Result

"Saving credentials" returns in a few seconds and the new balances show up in the portfolio right away; the connector becomes fully trade-ready ~80s later in the background.

Test plan

1. Add a Hyperliquid (perp + spot) credential via the API → response returns in seconds; balances appear on the next portfolio read.
2. Within the background-finish window, confirm the connector is present with balances; after ~80s confirm it is trade-ready (trading rules / markets loaded).
3. Add a bad key → still fails fast and the credential is deleted.
4. Add then immediately delete a credential → it does not reappear after the background finish completes.

🤖 Generated with Claude Code