[DeadCode] Add RemoveTestsOverriddenPrivateMethodParameterRector

[TomasVotruba]

Removes a private method parameter whose value is thrown away by a direct assign, before the parameter is ever read.

Scoped to PHPUnit test classes for now — the common case is a test helper that takes a param and immediately replaces it with a mock. Scope can widen later once the rule proves itself.

 use PHPUnit\Framework\TestCase;

 final class SomeTest extends TestCase
 {
     public function test()
     {
-        $this->createUser(new User());
+        $this->createUser();
     }

-    private function createUser($user)
+    private function createUser()
     {
         $user = $this->createMock(User::class);

         return $user;
     }
 }

Docblock goes with the parameter:

-    /**
-     * @param \PHPUnit\Framework\MockObject\MockObject&User $user
-     */
-    private function prepareCommonMocks(Event $event, MockObject $user): void
+    private function prepareCommonMocks(Event $event): void
     {
         $user = $this->createMock(User::class);
         // ...
     }

All in-class caller args are updated too.

How it decides

Only the first mention of the parameter in the method body counts. It must be a plain $param = ...; expression statement at the top level of the method, and the parameter must not appear on the right side.

Skipped:

case	why
class is not a PHPUnit test case	out of scope for now
echo $value; $value = 5;	read before override
if (...) { $value = 5; }	override is conditional
$value = $value ?: 5;	param read on the right side
&$value	by-ref writes back to the caller
variadic / promoted param	used by design
compact(), extract(), func_get_args(), func_num_args(), func_get_arg(), get_defined_vars(), $$name	value readable without naming the variable
public/protected method	out of scope, caller set is unknown
__construct()	called via new, not covered by caller args cleanup
first-class callable, unpacked or mismatched named args at call site	caller args cannot be rewritten safely

Notes

Caller args + @param removal is now shared between this rule and RemoveUnusedPrivateMethodParameterRector via a new PrivateMethodParamRemover; the existing rule keeps its behaviour, its fixtures are unchanged.

Rule is added to DeadCodeLevel::RULES.