Lend provider blocklist and abstraction gaps

## Summary

The lend provider base class has several gaps in allowlist/blocklist enforcement and duplicated logic that should be shared with swap providers.

## Blocklist is non-functional

`marketBlocklist` is defined in `LendProviderConfig` but **never checked anywhere** in `LendProvider`. A developer could configure a blocklist and it silently does nothing.

- `validateConfigSupported()` only checks the allowlist (LendProvider.ts:233-255)
- `filterMarketConfigs()` only filters by chainId/asset, not allowlist or blocklist
- `getMarkets()` doesn't enforce allowlist when custom markets array is provided

## Shared logic to reuse from swap refactor

After the swap provider refactor (PR #311), shared utilities exist that lend providers should adopt:

- **`swap/core/markets.ts`** — `expandMarkets`, `findMarket`, `assetPairs` patterns can inform equivalent lend utilities
- **`utils/approve.ts`** — `buildErc20ApprovalTx`, `buildApprovalTxIfNeeded`, `getApprovalDeficit` — `LendProvider.buildApprovalTx` should delegate to `buildErc20ApprovalTx` (already done)
- **`supportedChainIds()` 3-level intersection** — identical code in both base classes, should be extracted to a shared utility

## What to do

1. Implement blocklist checking in `LendProvider.validateConfigSupported()`
2. Enforce allowlist/blocklist in `getMarkets()` and `getMarket()`
3. Extract shared `supportedChainIds()` filtering to a utility both base classes use
4. Ensure `filterMarketConfigs()` respects both allowlist and blocklist

Start in LendProvider.ts, especially validateConfigSupported(), filterMarketConfigs(), getMarkets(), and getMarket(), then compare the shared patterns in swap/core/markets.ts and utils/approve.ts. Done means allowlists and blocklists are enforced consistently and supportedChainIds() filtering is shared by both base classes without changing approval behavior.

Written by the indexing model from the issue text.