test(compiler): 添加空合并赋值操作符测试用例并优化编译逻辑

- 添加 assign_coalesce_002.phpt 测试文件验证空合并赋值功能
- 在 AstNodeType 中新增 isConstFetch 和 isAssignOp 辅助方法
- 优化 CompilerBase 中的空合并赋值编译逻辑，使用三元运算符替代 if 语句
- 修复局部变量检测和类型推断相关问题

src/Php/AstNodeType.php

@@ -78,4 +78,14 @@ trait AstNodeType
     {
         return $expr instanceof Expr\Match_;
     }
+
+    protected function isConstFetch(NodeAbstract $expr): bool
+    {
+        return $expr instanceof Expr\ConstFetch;
+    }
+
+    protected function isAssignOp(NodeAbstract $expr): bool
+    {
+        return $expr instanceof Node\Expr\AssignOp or $expr instanceof Node\Expr\Assign;
+    }
 }

src/Php/CompilerBase.php

@@ -3864,9 +3864,7 @@ class CompilerBase extends \PhpAot\Core\Translator
         if ($this->isVarExpr($expr->var) and !$this->hasVar($var)) {
             $this->addLocalVar($var, $this->detectExprType($expr->expr));
         }
-        return 'if (!' . $isset . ') {' . PHP_EOL .
-            $this->getIndent() . $var . ' = ' . $right . ';' . PHP_EOL .
-            '}' . PHP_EOL;
+        return '(' . $isset . '?' . $var . ':(' . $var . ' = ' . $right . '))';
     }
 
     protected function isReturnStmtInLastLine(array $stmts): bool

tests/aot/assign_coalesce_002.phpt

@@ -0,0 +1,39 @@
+--TEST--
+assign coalesce 002
+--FILE--
+<?php
+function case1() {
+    $context = new ArrayObject();
+    $context['session'] = 'world';
+    return  $context['session'] ??= 'hello';
+}
+
+function case2() {
+    $context = new ArrayObject();
+    return  $context['session'] ??= 'hello';
+}
+
+function case3() {
+    $context = [];
+    $context['session'] = 'world';
+    return  $context['session'] ??= 'hello';
+}
+
+function case4() {
+    $context = [];
+    return  $context['session'] ??= 'hello';
+}
+
+function main() {
+    error_reporting(E_ERROR);
+    var_dump(case1());
+    var_dump(case2());
+    var_dump(case3());
+    var_dump(case4());
+}
+?>
+--EXPECT--
+string(5) "world"
+string(5) "hello"
+string(5) "world"
+string(5) "hello"