Refactor AssertJ assertions into more idiomatic ones

This commit refactors some AssertJ assertions into more idiomatic and
readable ones. Using the dedicated assertion instead of a generic one
will produce more meaningful error messages. 

For instance, consider collection size:
```
// expected: 5 but was: 2
assertThat(collection.size()).equals(5);
// Expected size: 5 but was: 2 in: [1, 2]
assertThat(collection).hasSize(5);
```

Closes gh-30104

spring-context-indexer/src/test/java/org/springframework/context/index/processor/CandidateComponentsIndexerTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -222,7 +222,7 @@ class CandidateComponentsIndexerTests {
 		for (Class<?> c : classes) {
 			assertThat(metadata).has(Metadata.of(c, Component.class));
 		}
-		assertThat(metadata.getItems()).hasSize(classes.length);
+		assertThat(metadata.getItems()).hasSameSizeAs(classes);
 	}
 
 	private void testSingleComponent(Class<?> target, Class<?>... stereotypes) {