Fix array_key_exists() with all int literal keys (#5197)

When checking code like the following:

```
<?php

function checkNegated(string $key): void {
    $arr = [
        0 => "foo",
        1 => "bar",
    ];

    if (!array_key_exists($key, $arr)) {
        printf("not found\n");
    }
}

function check(string $key): void {
    $arr = [
        0 => "foo",
        1 => "bar",
    ];

    if (array_key_exists($key, $arr)) {
        printf("found\n");
    }
}
```

the `if` in `checkNegated` would cause:

```
ERROR: RedundantCondition - 9:10 - Type string for $key is never =int(0)
```

This happens when the array keys are all int literals, but the "needle"
is a string.

`array_key_exists()` uses a loose equality comparison, but the generated
assertions for this specific case
(`AssertionFinder::getArrayKeyExistsAssertions`) was generating strict
equality clauses. This commit fixes it by changing the generated clause
from `=` to `~`.

src/Psalm/Internal/Analyzer/Statements/Expression/AssertionFinder.php

@@ -3770,7 +3770,7 @@ class AssertionFinder
                         }
                     } elseif ($key_type->allIntLiterals() && !$key_type->possibly_undefined) {
                         foreach ($key_type->getLiteralInts() as $array_literal_type) {
-                            $literal_assertions[] = '=' . $array_literal_type->getAssertionString();
+                            $literal_assertions[] = '~' . $array_literal_type->getAssertionString();
                         }
                     }
                 }

tests/TypeReconciliation/ArrayKeyExistsTest.php

@@ -327,6 +327,34 @@ class ArrayKeyExistsTest extends \Psalm\Tests\TestCase
                         return 1;
                     }'
             ],
+            'comparesStringAndAllIntKeysCorrectly' => [
+                '<?php
+                    /**
+                     * @param array<1|2|3, string> $arr
+                     * @return bool
+                     */
+                    function checkArrayKeyExistsComparison(array $arr, string $key): bool
+                    {
+                        if (array_key_exists($key, $arr)) {
+                            return true;
+                        }
+                        return false;
+                    }'
+            ],
+            'comparesStringAndAllIntKeysCorrectlyNegated' => [
+                '<?php
+                    /**
+                     * @param array<1|2|3, string> $arr
+                     * @return bool
+                     */
+                    function checkArrayKeyExistsComparisonNegated(array $arr, string $key): bool
+                    {
+                        if (!array_key_exists($key, $arr)) {
+                            return false;
+                        }
+                        return true;
+                    }'
+            ],
         ];
     }