Explore on-the-fly fiscal year parameter conversion

Context

PR #1435 fixes the fiscal year parameter conversion range by extending it from 2015-2025 to 2015-2040. This is a simple fix but requires pre-computing values for all 26 years at startup.

Proposed Enhancement

Explore converting fiscal year parameters on-the-fly rather than pre-computing them. This would:

1. Remove the year range limit entirely - No need to extend the range for future projections
2. Potentially improve startup time - No pre-computation needed for all years
3. Simplify the code - Just convert January 1 queries to April 30 dynamically

Attempted Implementation

An initial attempt at this approach was made in PR #1434 by:

• Adding convert_instant_to_fiscal_year() function
• Overriding get_parameters_at_instant() in CountryTaxBenefitSystem
• Making convert_to_fiscal_year_parameters() a no-op

However, this broke 24 existing tests because it changed parameter lookup behavior for ALL dates, not just annual queries.

PR #1434 contains the work-in-progress on-the-fly implementation - it can be used as a starting point.

Investigation Needed

• Understand why the on-the-fly approach broke existing tests
• Determine if on-the-fly conversion can be safely implemented
• Consider performance implications
• Evaluate backward compatibility concerns

References

• PR #1435: Simple fix (extend range to 2040) ✅
• PR #1434: On-the-fly approach (work-in-progress implementation for this issue)

Start with the on-the-fly implementation in PR #1434 and inspect convert_instant_to_fiscal_year(), CountryTaxBenefitSystem.get_parameters_at_instant(), and convert_to_fiscal_year_parameters(). Run the existing test suite to reproduce the 24 failures, then determine whether conversion can remove the year range without changing non-annual parameter lookups; done means preserving existing behavior while supporting future fiscal years.