adapitve spi should invoke `clazz.getConstructor()` to guarantee usability

adapitve spi should invoke `clazz.getConstructor()` to guarantee usability

> 自适应spi应该调用`clazz.getConstructor()`来保证可用性

in method `org.apache.dubbo.common.extension.ExtensionLoader#loadClass`

For normal SPI, we make sure we don't cache unavailable extension point implementations by calling 'clazz.getconstructor ()', but we don't do this for adaptive extensions

```java
private void loadClass(Map> extensionClasses, java.net.URL resourceURL, Class clazz, String name,
boolean overridden) throws NoSuchMethodException {
if (!type.isAssignableFrom(clazz)) {
throw new IllegalStateException("Error occurred when loading extension class (interface: " +
type + ", class line: " + clazz.getName() + "), class "
+ clazz.getName() + " is not subtype of interface.");
}
if (clazz.isAnnotationPresent(Adaptive.class)) {
cacheAdaptiveClass(clazz, overridden);
} else if (isWrapperClass(clazz)) {
cacheWrapperClass(clazz);
} else {
clazz.getConstructor();
if (StringUtils.isEmpty(name)) {
name = findAnnotationName(clazz);
if (name.length() == 0) {
throw new IllegalStateException("No such extension name for the class " + clazz.getName() + " in the config " + resourceURL);
}
}

String[] names = NAME_SEPARATOR.split(name);
if (ArrayUtils.isNotEmpty(names)) {
cacheActivateClass(clazz, names[0]);
for (String n : names) {
cacheName(clazz, n);
saveInExtensionClass(extensionClasses, clazz, n, overridden);
}
}
}
}
```

I think the following makes more sense

```java
private void loadClass(Map> extensionClasses, java.net.URL resourceURL, Class clazz, String name,
boolean overridden) throws NoSuchMethodException {
if (!type.isAssignableFrom(clazz)) {
throw new IllegalStateException("Error occurred when loading extension class (interface: " +
type + ", class line: " + clazz.getName() + "), class "
+ clazz.getName() + " is not subtype of interface.");
}

if (isWrapperClass(clazz)) {
cacheWrapperClass(clazz);
return;
}
// guarantee usability of spi
clazz.getConstructor();
if (clazz.isAnnotationPresent(Adaptive.class)) {
cacheAdaptiveClass(clazz, overridden);
return;
}

if (StringUtils.isEmpty(name)) {
name = findAnnotationName(clazz);
if (name.length() == 0) {
throw new IllegalStateException("No such extension name for the class " + clazz.getName() + " in the config " + resourceURL);
}
}

String[] names = NAME_SEPARATOR.split(name);
if (ArrayUtils.isNotEmpty(names)) {
cacheActivateClass(clazz, names[0]);
for (String n : names) {
cacheName(clazz, n);
saveInExtensionClass(extensionClasses, clazz, n, overridden);
}
}
}
```

Start at org.apache.dubbo.common.extension.ExtensionLoader#loadClass and compare the existing constructor check for normal SPI classes with the adaptive-extension branch. Verify how adaptive classes without a usable public constructor are handled, then add or update focused tests so unusable adaptive extensions are rejected while valid ones remain loadable.

Written by the indexing model from the issue text.