Fix InvalidCastException in projectable constructors when a property name collides with a member-access selector name

[Copilot]

Block-bodied [Projectable] constructors that assign a property (e.g. Name) and later access a same-named member on an unrelated parameter (e.g. category.Name) caused the generator to throw InvalidCastException and silently fail to emit source.

[Projectable]
public InventoryEntry(Item item, ItemCategory category) {
    Name = item.Name;
    CategoryName = category.Name; // crashed the generator
}

Root cause

ConstructorBodyConverter's syntax substitutors (ParameterSubstitutor, LocalVariableSubstitutor, AssignedPropertySubstitutor) override VisitIdentifierName to inline known parameters/locals/assigned-properties by name. None of them guarded against visiting the Name part of a MemberAccessExpressionSyntax (e.g. the Name in category.Name). Once Name had been assigned earlier in the body, AssignedPropertySubstitutor would replace that .Name selector with a ParenthesizedExpressionSyntax — invalid where a SimpleNameSyntax is required — triggering the cast exception when the tree was rebuilt.

Fix

• Added VisitMemberAccessExpression overrides to all three substitutor classes so they only ever rewrite the target Expression of a member access, never the Name selector.
• AssignedPropertySubstitutor still handles its intentional @this.PropName and bare-identifier substitution cases explicitly; only the unmatched fallback path was constrained.

Tests

• Added ProjectableConstructor_MemberAccessNameCollidesWithAssignedPropertyName generator test reproducing the issue, with a verified snapshot confirming Name = item.Name, CategoryName = category.Name is emitted correctly.

• Fixes Source generator fails with InvalidCastException #220