Handle ternary, switch, and cast expressions in AOT CCW discovery

[Sergio0694]

Fixes #1947.

Problem

The AOT source generator's CCW lookup table discovery (GetVtableAttributesToAddOnLookupTable in AotOptimizer.cs) called GetTypeInfo directly on the initializer / assignment / return / argument expressions it inspects. For a conditional (ternary) or switch expression, GetTypeInfo only yields the common target type of all branches (typically object or an interface), and for a cast expression it yields the cast target type. So the concrete types actually being boxed or cast were never discovered and got no CCW vtable entries or generic instantiations, failing at runtime under Native AOT.

// Nothing here was discovered: 'List<int>' and 'List<string>' both got no CCW vtable
object obj = GetFlag() ? new List<int>() : (object)new List<string>();

Fix

Add an AddVtableAttributesForExpression helper that looks through parenthesized, cast, conditional (ternary), and switch expressions to reach the concrete leaf expressions that flow into the target, and route every expression-based call site through it. For casts the target type is still processed directly, so existing (List<int>)value discovery is preserved while the operand ((object)new List<string>()) is now also discovered.

Testing

• Source generator unit tests (src/Tests/SourceGeneratorTest/AotOptimizerTests.cs): run WinRTAotSourceGenerator over ternary, switch, and cast boxing and assert the concrete types are registered in the generated CCW vtable lookup table. The three positive tests fail without the fix and pass with it; a control test guards against discovering types that aren't actually boxed.
• AOT CCW functional test (src/Tests/FunctionalTests/CCW/Program.cs): box List<T> instances only reachable through a ternary or switch expression and validate, under Native AOT, that their CCWs expose the expected IVector<T> vtable via the reported runtime class name.

Verified locally: SourceGeneratorTest (68 tests) and DiagnosticTests (364 tests) pass, and both the Roslyn 4.8 and Roslyn 4.12 generator variants build cleanly.

Grouped as three commits: the generator fix, the AOT functional test, and the unit tests.