Ensure SAL/C++ attributes on fields generate a CppAttributeList

[tannergooding]

Under generate-cpp-attributes, a C++ attribute on a parameter was surfaced as [CppAttributeList("...")], but the same attribute on a struct field was silently dropped. CppAttributes was only ever wired for ParameterDesc/BeginParameter, never for FieldDesc/BeginField, so SAL buffer annotations (e.g. _Field_size_full_, _Field_size_) surfaced as annotate attributes never reached the field's CppAttributeList.

This mirrors the existing parameter path for fields:

• Abstractions/FieldDesc.cs — add IEnumerable<string>? CppAttributes.
• PInvokeGenerator.VisitDecl.cs VisitFieldDecl — populate it from fieldDecl.Attrs when GenerateCppAttributes is set (identical shape to the parameter path).
• CSharp/CSharpOutputBuilder.VisitDecl.cs BeginField — emit it via AddCppAttributes, right after the NativeTypeName block so ordering matches parameters (CppAttributeList before NativeAnnotation).

CppAttributeList is the opt-in, full-fidelity dump of the raw C++ attribute list (all kinds, ^-joined, gated on generate-cpp-attributes), distinct from the always-on NativeAnnotation that surfaces only annotate spellings. Only the former was missing on fields.

Example (field now matches the equivalent parameter):

#define _Field_size_full_(x) __attribute__((annotate("_Field_size_full_(" #x ")")))
struct MyStruct { int count; _Field_size_full_(count) int* data; };

[CppAttributeList("_Field_size_full_(count)")]
[NativeAnnotation("_Field_size_full_(count)")]
public int* data;

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Adds a standalone regression test (SalFieldAttributeTest, [Platform("win")]) under generate-cpp-attributes asserting the field's CppAttributeList. No existing goldens changed (no prior test enabled generate-cpp-attributes). Full ClangSharp.PInvokeGenerator.UnitTests run: 3725 passed, 0 failed, 0 warnings.

The indirect-field accessor path (nested-anonymous accessor properties) is intentionally left out of scope.

Fixes #444

Co-authored-by: Copilot App 223556219+Copilot@users.noreply.github.com