fix(postgrest): fix field promotion null-safety issues in PostgrestBuilder

Use local variable extraction to enable null promotion for final fields
in PostgrestBuilder that could not be promoted due to class inheritance.
This fixes dart test compilation errors affecting all test files.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: grdsdev

packages/postgrest/lib/src/postgrest_builder.dart

@@ -148,12 +148,13 @@ class PostgrestBuilder<T, S, R> implements Future<T> {
     // X-Retry-Count, etc.).
     final execHeaders = {..._headers};
 
-    if (_count != null) {
+    final count = _count;
+    if (count != null) {
       if (execHeaders['Prefer'] != null) {
         final oldPreferHeader = execHeaders['Prefer'];
-        execHeaders['Prefer'] = '$oldPreferHeader,count=${_count.name}';
+        execHeaders['Prefer'] = '$oldPreferHeader,count=${count.name}';
       } else {
-        execHeaders['Prefer'] = 'count=${_count.name}';
+        execHeaders['Prefer'] = 'count=${count.name}';
       }
     }
 
@@ -173,12 +174,13 @@ class PostgrestBuilder<T, S, R> implements Future<T> {
         );
       }
 
-      if (_schema == null) {
+      final schema = _schema;
+      if (schema == null) {
         // skip
       } else if (method == _HttpMethod.get || method == _HttpMethod.head) {
-        execHeaders['Accept-Profile'] = _schema;
+        execHeaders['Accept-Profile'] = schema;
       } else {
-        execHeaders['Content-Profile'] = _schema;
+        execHeaders['Content-Profile'] = schema;
       }
       if (method != _HttpMethod.get && method != _HttpMethod.head) {
         execHeaders['Content-Type'] = 'application/json';
@@ -284,8 +286,9 @@ class PostgrestBuilder<T, S, R> implements Future<T> {
           body = response.body;
         } else {
           try {
-            if ((response.contentLength ?? 0) > 10000 && _isolate != null) {
-              body = await _isolate.decode(response.body);
+            final isolate = _isolate;
+            if ((response.contentLength ?? 0) > 10000 && isolate != null) {
+              body = await isolate.decode(response.body);
             } else {
               body = jsonDecode(response.body);
             }
@@ -339,16 +342,17 @@ class PostgrestBuilder<T, S, R> implements Future<T> {
       }
       body as R;
 
-      if (_converter != null) {
-        converted = _converter(body);
+      final converter = _converter;
+      if (converter != null) {
+        converted = converter(body);
       } else {
         converted = body as S;
       }
 
-      if (_count != null && method != _HttpMethod.head) {
+      if (count != null && method != _HttpMethod.head) {
         return PostgrestResponse<S>(
           data: converted,
-          count: count!,
+          count: count,
         ) as T;
       } else {
         return converted as T;
@@ -400,17 +404,18 @@ class PostgrestBuilder<T, S, R> implements Future<T> {
   ) {
     if (error.details is String &&
         error.details.toString().contains('Results contain 0 rows')) {
+      final converter = _converter;
       if (_count != null &&
           response.request!.method != _HttpMethod.head.value) {
-        if (_converter != null) {
-          return PostgrestResponse<S>(data: _converter(null as R), count: 0)
+        if (converter != null) {
+          return PostgrestResponse<S>(data: converter(null as R), count: 0)
               as T;
         } else {
           return null as T;
         }
       } else {
-        if (_converter != null) {
-          return _converter(null as R) as T;
+        if (converter != null) {
+          return converter(null as R) as T;
         } else {
           return null as T;
         }