Fix nullable reference conversion elision for user-defined conversions

[marinasundstrom]

Motivation

• Prevent elision of conversions that should call user-defined or extension op_Implicit when targeting nullable reference unions.
• The compiler previously returned the original expression for Nullable<T> (reference underlying) too broadly, which suppressed emitted conversion calls.
• This led to missing/incorrect IL for conversions like the implicit operator in OptionExtensions<T> and runtime failures when the conversion was expected to run.

Description

• Tighten the guard in BlockBinder.ApplyConversion so the early-return for NullableTypeSymbol only occurs when conversion.IsReference and the expression type exactly equals the nullable underlying type using SymbolEqualityComparer.Default.Equals.
• This ensures user-defined and extension conversion operators (including op_Implicit) are considered and emitted for nullable reference targets instead of being elided.
• The change is implemented in src/Raven.CodeAnalysis/Binder/BlockBinder.cs by refining the if (targetType is NullableTypeSymbol ...) condition.

Testing

• Ran the code generation steps (NodeGenerator, BoundNodeGenerator, DiagnosticsGenerator) before building as required by repository guidelines.
• Built the solution with dotnet build --property WarningLevel=0, which completed successfully.
• Ran dotnet test test/Raven.CodeAnalysis.Tests /property:WarningLevel=0 -v minimal, which exercised the test suite but encountered many preexisting and unrelated failures; tests did not all pass.
• Manual verification: inspected emitted IL/behavior for the reported conversion scenario and adjusted the binding logic to allow conversion emission (build verified the change introduced no compilation errors).

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Codex Task