[13.x] Add flushState to FormRequest to reset global strict mode between tests

[sumaiazaman]

Summary

PR #59430 added FormRequest::failOnUnknownFields() which sets a static $globalFailOnUnknownFields flag. This flag persists across tests because FormRequest has no flushState() method and isn't included in the test lifecycle teardown.

The Bug

class FirstTest extends TestCase
{
    public function testStrictMode()
    {
        FormRequest::failOnUnknownFields();
        // test strict validation...
    }
}

class SecondTest extends TestCase
{
    public function testNormalMode()
    {
        // This test ALSO runs in strict mode because
        // $globalFailOnUnknownFields is still true!
    }
}

The Fix

1. Add flushState() method to FormRequest that resets $globalFailOnUnknownFields
2. Call it in InteractsWithTestCaseLifecycle teardown alongside all other flushState() calls

Context

Every other class with static state has flushState() called in the test teardown:

• ConvertEmptyStringsToNull::flushState()
• TrimStrings::flushState()
• EncryptCookies::flushState()
• PreventRequestForgery::flushState()
• Validator::flushState()
• etc.

FormRequest was missed because failOnUnknownFields() was just added in #59430.

Changes

• src/Illuminate/Foundation/Http/FormRequest.php — Add flushState() method
• src/Illuminate/Foundation/Testing/Concerns/InteractsWithTestCaseLifecycle.php — Call FormRequest::flushState() in teardown