[CodeQuality] Verify assert method name exists on Assert class via reflection

[TomasVotruba]

AssertTrueFalseToSpecificMethodRector mapped file_exists/is_dir to assertFileNotExists()/assertDirectoryNotExists(). These were renamed in newer PHPUnit versions — the produced call no longer exists on the Assert class, breaking the test.

Now the rule verifies the target method exists on PHPUnit\Framework\Assert via reflection, and falls back to the current name when the legacy one is gone.

Before

$this->assertFalse(file_exists($file));

on PHPUnit 12 became:

-$this->assertFalse(file_exists($file));
+$this->assertFileNotExists($file); // method removed → fatal

After

-$this->assertFalse(file_exists($file));
+$this->assertFileDoesNotExist($file);

Renames covered:

legacy	current
assertFileNotExists	assertFileDoesNotExist
assertDirectoryNotExists	assertDirectoryDoesNotExist

When the legacy method still exists (older PHPUnit), it is kept as-is.