Replace @assert with proper exceptions in risk measure constructors

[alecloudenback]

Summary

The VaR, CTE, and WangTransform constructors use @assert for input validation:

function VaR(α::T) where {T}
    @assert 0 <= α < 1 "α of $α is not 0 ≤ α < 1"
    return new{T}(α)
end

@assert can be optimized away by the compiler (e.g. with --check-bounds=no), making it unreliable for public API validation. These should use throw(DomainError(...)) or throw(ArgumentError(...)).

The test file (test/risk_measures.jl) currently tests for AssertionError specifically, which would need updating.

Fix

• Replace @assert with explicit throw in VaR, CTE, WangTransform constructors
• Update tests from @test_throws AssertionError to the new exception type

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