[Draft] Fix fiscal year parameter handling (on-the-fly approach - needs work)

[MaxGhenis]

Summary

• Fix fiscal year parameter handling to use April 30 reference date for annual queries
• Ensures policies that change on April 6 (UK fiscal year start) are correctly reflected in simulations

Problem

Previously, convert_to_fiscal_year_parameters() only covered years 2015-2025 (range(2015, 2026)). When querying parameters for year 2026 or later, the system fell back to January 1 as the reference date instead of April 30.

This caused policies that change on April 6 (UK fiscal year start) to not be reflected in annual simulations. For example:

• The two-child limit repeal on April 6, 2026 would show as limit=2 for 2026 simulations instead of limit=infinity
• The salary sacrifice cap starting April 6, 2029 would not be reflected in 2029 simulations

Solution

1. Add convert_instant_to_fiscal_year() function that converts year-only and January 1 queries to April 30 of that year
2. Override get_parameters_at_instant() in CountryTaxBenefitSystem to apply this conversion on-the-fly
3. Make convert_to_fiscal_year_parameters() a no-op (deprecated) since the conversion is now done on-the-fly

This ensures:

• param("2026") returns the April 30, 2026 value (fiscal year 2026/27)
• Specific date queries like param("2026-04-05") are unchanged
• Works for any future year without needing to extend a range

Test plan

• Run new fiscal year parameter tests
• Verify existing tests pass
• CI passes

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