Fix initialInvestment type and add liteSurchargeAccruals field

[Copilot]

IBKR Flex XML uses initialInvestment="Yes" and includes liteSurchargeAccruals in equity summaries, but the parser expected decimal values for initialInvestment and was missing the liteSurchargeAccruals attribute entirely.

# Before: raises FlexParserError
parser.parse_data_element(ET.fromstring('<Trade initialInvestment="Yes" />'))
# Error: Can't convert 'Yes' to <class 'decimal.Decimal'>

# Before: raises FlexParserError  
parser.parse_data_element(ET.fromstring(
    '<EquitySummaryByReportDateInBase liteSurchargeAccruals="5.50" />'
))
# Error: EquitySummaryByReportDateInBase has no attribute 'liteSurchargeAccruals'

Changes

• Type correction: Changed initialInvestment from Optional[decimal.Decimal] to Optional[bool] in Trade, Lot, SymbolSummary, AssetSummary, Order, OptionEAE, and SecurityInfo classes

• Missing field: Added liteSurchargeAccruals: Optional[decimal.Decimal] to EquitySummaryByReportDateInBase

• Boolean parser: Extended to accept "Yes"/"No" in addition to existing "Y"/"N" values

• Test coverage: Added TradeInitialInvestmentTestCase and EquitySummaryLiteSurchargeAccrualsTestCase

Original prompt

Fix vroonhof/opensteuerauszug#106 which is caused by this library.

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