C#: Improve type attribution coverage on identifiers and declarations

[jkschneider]

Summary

• Adds JavaType attribution to ~30 identifier creation sites in the C# parser that were previously null, null
• Covers declaration names (class, method, constructor, property, event, enum, delegate), expression identifiers (VisitIdentifierName fieldType, FieldAccess name, MethodInvocation name), variable declaration names (fields, locals, parameters, for/foreach, catch, pattern match, lambda params), and type references (non-primitive predefined types, generic raw type identifiers)
• Adds FieldType(), TypeAndFieldType(), and RawType() methods to CSharpTypeMapping for reference resolution, combined Roslyn API calls, and unparameterized generic type lookup
• Caches JavaType.Variable instances by ISymbol in MapVariable for referential deduplication across declaration and reference sites
• Adds comprehensive test verifying class name type, method name type, property fieldType, field variable fieldType, and round-trip correctness

Test plan

• Existing parseWithTypeAttribution test passes
• New comprehensiveTypeAttribution test passes — verifies class/method/property/field type attribution
• All 50+ existing CSharpRpcTest round-trip tests pass (no regressions)
• All CSharpRecipeTest recipe tests pass
• C# solution builds with zero warnings