feat: Enhance currency handling and validation across scenarios

- Updated form template to prefill currency input with default value and added help text for clarity.
- Modified integration tests to assert more descriptive error messages for invalid currency codes.
- Introduced new tests for currency normalization and validation in various scenarios, including imports and exports.
- Added comprehensive tests for pricing calculations, ensuring defaults are respected and overrides function correctly.
- Implemented unit tests for pricing settings repository, ensuring CRUD operations and default settings are handled properly.
- Enhanced scenario pricing evaluation tests to validate currency handling and metadata defaults.
- Added simulation tests to ensure Monte Carlo runs are accurate and handle various distribution scenarios.

tests/test_financial.py

@@ -0,0 +1,162 @@
+from __future__ import annotations
+
+from datetime import date
+
+import pytest
+
+from services.financial import (
+    CashFlow,
+    PaybackNotReachedError,
+    internal_rate_of_return,
+    net_present_value,
+    normalize_cash_flows,
+    payback_period,
+)
+
+
+def test_normalize_cash_flows_with_dates() -> None:
+    base = date(2025, 1, 1)
+    period_length = 365.0 / 4
+    flows = [
+        CashFlow(amount=-1_000_000, date=base),
+        CashFlow(amount=350_000, date=date(2025, 4, 1)),
+        CashFlow(amount=420_000, date=date(2025, 7, 1)),
+    ]
+
+    normalised = normalize_cash_flows(flows, compounds_per_year=4)
+
+    assert normalised[0] == (-1_000_000.0, 0.0)
+    expected_second = (date(2025, 4, 1) - base).days / period_length
+    expected_third = (date(2025, 7, 1) - base).days / period_length
+
+    assert normalised[1][1] == pytest.approx(expected_second, rel=1e-6)
+    assert normalised[2][1] == pytest.approx(expected_third, rel=1e-6)
+
+
+def test_net_present_value_with_period_indices() -> None:
+    rate = 0.10
+    flows = [
+        CashFlow(amount=-1_000, period_index=0),
+        CashFlow(amount=500, period_index=1),
+        CashFlow(amount=500, period_index=2),
+        CashFlow(amount=500, period_index=3),
+    ]
+
+    expected = -1_000 + sum(500 / (1 + rate) **
+                            period for period in range(1, 4))
+
+    result = net_present_value(rate, flows)
+
+    assert result == pytest.approx(expected, rel=1e-9)
+
+
+def test_net_present_value_with_residual_value() -> None:
+    rate = 0.08
+    flows = [
+        CashFlow(amount=-100_000, period_index=0),
+        CashFlow(amount=30_000, period_index=1),
+        CashFlow(amount=35_000, period_index=2),
+    ]
+
+    expected = (
+        -100_000
+        + 30_000 / (1 + rate)
+        + 35_000 / (1 + rate) ** 2
+        + 25_000 / (1 + rate) ** 3
+    )
+
+    result = net_present_value(rate, flows, residual_value=25_000)
+
+    assert result == pytest.approx(expected, rel=1e-9)
+
+
+def test_internal_rate_of_return_simple_case() -> None:
+    flows = [
+        CashFlow(amount=-1_000, period_index=0),
+        CashFlow(amount=1_210, period_index=1),
+    ]
+
+    irr = internal_rate_of_return(flows, guess=0.05)
+
+    assert irr == pytest.approx(0.21, rel=1e-9)
+
+
+def test_internal_rate_of_return_multiple_sign_changes() -> None:
+    flows = [
+        CashFlow(amount=-500_000, period_index=0),
+        CashFlow(amount=250_000, period_index=1),
+        CashFlow(amount=-100_000, period_index=2),
+        CashFlow(amount=425_000, period_index=3),
+    ]
+
+    irr = internal_rate_of_return(flows, guess=0.2)
+
+    npv = net_present_value(irr, flows)
+
+    assert npv == pytest.approx(0.0, abs=1e-6)
+
+
+def test_internal_rate_of_return_requires_mixed_signs() -> None:
+    flows = [
+        CashFlow(amount=100_000, period_index=0),
+        CashFlow(amount=150_000, period_index=1),
+    ]
+
+    with pytest.raises(ValueError):
+        internal_rate_of_return(flows)
+
+
+def test_payback_period_exact_period() -> None:
+    flows = [
+        CashFlow(amount=-120_000, period_index=0),
+        CashFlow(amount=40_000, period_index=1),
+        CashFlow(amount=40_000, period_index=2),
+        CashFlow(amount=40_000, period_index=3),
+    ]
+
+    period = payback_period(flows, allow_fractional=False)
+
+    assert period == pytest.approx(3.0)
+
+
+def test_payback_period_fractional_period() -> None:
+    flows = [
+        CashFlow(amount=-100_000, period_index=0),
+        CashFlow(amount=80_000, period_index=1),
+        CashFlow(amount=30_000, period_index=2),
+    ]
+
+    fractional = payback_period(flows)
+    whole = payback_period(flows, allow_fractional=False)
+
+    assert fractional == pytest.approx(1 + 20_000 / 30_000, rel=1e-9)
+    assert whole == pytest.approx(2.0)
+
+
+def test_payback_period_raises_when_never_recovered() -> None:
+    flows = [
+        CashFlow(amount=-250_000, period_index=0),
+        CashFlow(amount=50_000, period_index=1),
+        CashFlow(amount=60_000, period_index=2),
+        CashFlow(amount=70_000, period_index=3),
+    ]
+
+    with pytest.raises(PaybackNotReachedError):
+        payback_period(flows)
+
+
+def test_payback_period_with_quarterly_compounding() -> None:
+    base = date(2025, 1, 1)
+    flows = [
+        CashFlow(amount=-120_000, date=base),
+        CashFlow(amount=35_000, date=date(2025, 4, 1)),
+        CashFlow(amount=35_000, date=date(2025, 7, 1)),
+        CashFlow(amount=50_000, date=date(2025, 10, 1)),
+    ]
+
+    period = payback_period(flows, compounds_per_year=4)
+
+    period_length = 365.0 / 4
+    expected_period = (date(2025, 10, 1) - base).days / period_length
+
+    assert period == pytest.approx(expected_period, abs=1e-6)