Fix CodeWriter field dedup to update references via MemberExpression

[Copilot]

CodeWriterDeclaration dedup (e.g. _scope → _scope0) only applied to the declaration site. References through FieldProvider.AsValueExpression wrote the original name via AppendRaw(MemberName), bypassing the declaration's actual name entirely.

// Before: declaration deduped, but reference still uses original name
private readonly string _scope0; // ✅ deduped
_scope = value;                  // ❌ wrong name

// After: reference resolves through CodeWriterDeclaration
private readonly string _scope0; // ✅ deduped
_scope0 = value;                 // ✅ correct

Changes

• MemberExpression: Added optional CodeWriterDeclaration? property. Write() delegates to writer.AppendDeclarationReference(Declaration) when present, which resolves the actual deduped name via scope lookup without accidentally creating new declarations.
• CodeWriter.AppendDeclarationReference(): New method that resolves a declaration's deduped name when it exists in scope, but falls back to RequestedName otherwise. Unlike Append(CodeWriterDeclaration), it never calls WriteDeclaration, so it cannot accidentally register new deduped names in the wrong scope.
• FieldProvider: Implicit operator to MemberExpression now sets Declaration = field.Declaration. Update() syncs _asMember.Declaration when name changes (since _declaration is reset).
• Tests:
  • FieldDedupUpdatesReferencesViaMemberExpression — writes two fields with the same name, verifies the reference to the second field uses the deduped name.
  • FieldReferenceBeforeDeclarationUsesRequestedName — writes a field reference before its declaration, verifies it uses the original name without creating a spurious deduped name.

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[jorgerangel-msft]

@copilot something is not right here. The generated collectionresults definition is incorrectly referencing a field that doesn't exist Azure/azure-sdk-for-net#57468

[Copilot]

@copilot something is not right here. The generated collectionresults definition is incorrectly referencing a field that doesn't exist Azure/azure-sdk-for-net#5...

Fixed in 2504336. The root cause was that MemberExpression.Write() called writer.Append(Declaration), and CodeWriter.Append(CodeWriterDeclaration) has a fallthrough path that calls WriteDeclaration when the declaration hasn't been registered in the current scope yet. This turned field references into accidental declarations, causing the deduplication suffix (_token → _token0) on references while the actual field declarations kept their original names.

Added CodeWriter.AppendDeclarationReference() which resolves the deduped name when the declaration exists in scope, but falls back to RequestedName otherwise — it never creates new declarations. Also added a regression test FieldReferenceBeforeDeclarationUsesRequestedName.

[jorgerangel-msft]

@copilot this looks almost identical to the existing Append method. We should just update the existing method with an optional bool parameter