Rename tau parameter to WithdrawTax in RiskyContribModel

[Copilot]

This PR addresses the issue where the tau parameter in RiskyContribModel used a symbolic name instead of an economic name, which goes against HARK's naming conventions.

Changes Made

• Renamed parameter: Changed tau to WithdrawTax throughout the RiskyContribModel
• Updated documentation: Improved parameter descriptions from "Tax rate of some kind" to "Tax rate on risky asset withdrawals"
• Function signatures: Updated all function definitions and calls (27 references total)
• Tests: Updated test file to use the new parameter name
• Default value: Added "WithdrawTax": [0.0] to default parameters
• Class metadata: Updated time_vary_ list to include "WithdrawTax"

Economic Meaning

The parameter represents a tax rate applied to withdrawals from risky assets to risk-free assets. The new name WithdrawTax clearly conveys this economic meaning and follows HARK's convention of using descriptive names like DiscFac, PermGroFac, BoroCnstArt, etc.

Files Modified

• HARK/ConsumptionSaving/ConsRiskyContribModel.py - Main model implementation
• tests/ConsumptionSaving/test_ConsRiskyContribModel.py - Test file

The changes are minimal and surgical, preserving all existing functionality while making the parameter name economically meaningful.

Fixes #1539.

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