fix(check-order-status): fall back to 12s block time for chains unsupported by sdk-core

[aayala-uniswap]

Problem

check-order-status is crashing on every retry for orders on Sepolia (chainId 11155111) — and potentially any other chain in SUPPORTED_CHAINS not yet registered in @uniswap/sdk-core:

```
Error: getAverageBlockTimeSecs: unsupported chainId 11155111;
register it in chains.ts before use
at AVERAGE_BLOCK_TIME (check-order-status/util.ts:251)
```

This was observed in beta CloudWatch logs (GoudaServicebetaCheckOrd) hitting repeatedly today (June 3), triggered by the order posted at 22:25 UTC. The step function retries indefinitely, never reaching a terminal state.

Fix

Wrap AVERAGE_BLOCK_TIME in a try/catch. Falls back to 12 seconds (Ethereum PoS slot time) for any chain the SDK doesn't know about yet, so status tracking degrades gracefully instead of looping forever.

export const AVERAGE_BLOCK_TIME = (chainId: ChainId): number => {
  try {
    return getAverageBlockTimeSecs(chainId)
  } catch {
    return 12
  }
}

Why 12s?

Sepolia uses the same 12s PoS slot time as Ethereum mainnet. It's also a safe conservative default for any other unknown chain — block-number arithmetic will be slightly off but won't crash.

Test plan

• No more getAverageBlockTimeSecs: unsupported chainId 11155111 errors in GoudaServicebetaCheckOrd logs after deploy
• Existing unit tests pass (yarn test)
• Order posted on Sepolia reaches a terminal state (filled / expired) rather than looping forever

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