fix: properly await API call and handle validation errors gracefully

[maxwellyoung]

Summary

Fixes #104 - The MCP server crashed on Xero API validation errors (HTTP 400) due to unhandled promise rejections.

Root Causes & Fixes

1. Missing await in updateBankTransaction

The API call at line 49 was not awaited, causing promises to be fire-and-forget:

// Before (bug)
xeroClient.accountingApi.updateBankTransaction(...);
return bankTransaction;  // Returns input, not actual response

// After (fix)
const response = await xeroClient.accountingApi.updateBankTransaction(...);
return response.body.bankTransactions?.[0];  // Returns actual API response

2. No extraction of Xero validation errors

Enhanced formatError to handle 400 status codes and extract meaningful validation messages from Xero's error response structure:

// Extracts from: error.response.body.Elements[0].ValidationErrors[].Message

Changes

• src/handlers/update-xero-bank-transaction.handler.ts: Added missing await, return actual API response
• src/helpers/format-error.ts: Added 400 status handling with validation error extraction

Test plan

• Build passes
• Lint passes
• Manual test: Send invalid data to trigger 400 error, verify graceful error message instead of crash