Added:
- 5 option exercise methods: option_exercise_check, get_option_exercise_positions,
  submit_option_exercise (Result<Option<bool>>), get_option_exercise_records,
  cancel_option_exercise (Result<Option<bool>>)
- TradeClient::with_secret_key() constructor for institutional accounts
- ClientConfig::secret_key field (readable from .properties file)
- All option exercise request struct fields are Option<T> with skip_serializing_if
- option_exercise_submit/cancel added to TRADE_OPERATIONS retry exclusion list

Fixed:
- decode_value: preserve original error in fallback branch instead of repeating from_value