Handle explicit type conversions as potential nil producers (#166)

[nickita-khylkouski]

Fixes #166

Problem

NilAway did not handle explicit type conversions (like (*Struct)(nil) or StructPtr(nil)), resulting in false negatives. These type conversions can produce nil values and should be treated as potential nil producers.

Example:

var explicit *Struct
explicit = (*Struct)(nil)
_ = explicit.Field // PANICS but NilAway missed this

explicitWithoutParens := StructPtr(nil)
_ = explicitWithoutParens.Field // PANICS but NilAway missed this

Solution

Added a check in ParseExprAsProducer to detect when a *ast.CallExpr represents an explicit type conversion (using TypesInfo.Types[expr.Fun].IsType()). When detected:

1. If the target type bars nilness (e.g., int, functions), treat the result as non-nil
2. Otherwise, pass-through the nilability of the converted expression since the conversion can be nil if and only if the argument is nil and the type allows nil

Testing

[x] All linting checks pass
[x] Full test suite passes
[x] New test cases added for:

• Explicit type conversions with parens ((*Struct)(nil))
• Named type conversions without parens (StructPtr(nil))
• Conversions with non-nil values (no false positive)
• Conversions to non-nilable types (no false positive)