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.

services/currency.py

@@ -0,0 +1,43 @@
+from __future__ import annotations
+
+"""Utilities for currency normalization within pricing and financial workflows."""
+
+import re
+from dataclasses import dataclass
+
+VALID_CURRENCY_PATTERN = re.compile(r"^[A-Z]{3}$")
+
+
+@dataclass(frozen=True)
+class CurrencyValidationError(ValueError):
+    """Raised when a currency code fails validation."""
+
+    code: str
+
+    def __str__(self) -> str:  # pragma: no cover - dataclass repr not required in tests
+        return f"Invalid currency code: {self.code!r}"
+
+
+def normalise_currency(code: str | None) -> str | None:
+    """Normalise currency codes to uppercase ISO-4217 values."""
+
+    if code is None:
+        return None
+    candidate = code.strip().upper()
+    if not VALID_CURRENCY_PATTERN.match(candidate):
+        raise CurrencyValidationError(candidate)
+    return candidate
+
+
+def require_currency(code: str | None, default: str | None = None) -> str:
+    """Return normalised currency code, falling back to default when missing."""
+
+    normalised = normalise_currency(code)
+    if normalised is not None:
+        return normalised
+    if default is None:
+        raise CurrencyValidationError("<missing currency>")
+    fallback = normalise_currency(default)
+    if fallback is None:
+        raise CurrencyValidationError("<invalid default currency>")
+    return fallback