Get detailed information about a specific ad account.
Args:
account_id: Meta Ads account ID (format: act_XXXXXXXXX)
access_token: Meta API access token (optional - will use cached token if not provided)
fields: Optional comma-separated Graph API fields to return. When provided,
replaces the default field set. Useful for fetching extras like
funding_source_details, spend_cap, is_prepay_account, min_daily_budget,
disable_reason, capabilities. Default fields:
id, name, account_id, account_status, amount_spent, balance, currency,
age, business_city, business_country_code, timezone_name.
For prepaid accounts (is_prepay_account=true, common in Brazil), the
Business Manager "available balance" is the sum of funding_source_details
STORED_BALANCE entries plus coupons — the `balance` field alone is the
amount due to be billed, not the available pre-paid funds.