🧹 Centralize error JSON generation in Response class

[rowan-m]

🎯 What: Duplicated hardcoded JSON error strings for E_CONNECTION_FAILED and E_BAD_RESPONSE were removed from multiple RequestMethod implementations and replaced with a central Response::errorJson() method.
💡 Why: This reduces code duplication and ensures consistent error response formatting across all request methods. It also makes it easier to change the error response format in the future if needed.
✅ Verification: A standalone PHP verification script was used to ensure that Response::errorJson() generates the correct JSON and that it is properly parsed by Response::fromJson().
✨ Result: Improved code maintainability and reduced duplication by centralizing error formatting logic.

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PR created automatically by Jules for task 13806251128490405200 started by @rowan-m

[google-labs-jules]

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[rowan-m]

Run composer run test and make sure you address the test failures.

[google-labs-jules]

Run composer run test and make sure you address the test failures.

I have updated the code to address the test failures and PHP version compatibility issues. Specifically, I removed the modern PHP type hints to maintain support for PHP 5.6 and reverted to manual string concatenation in Response::errorJson to ensure the exact whitespace formatting that existing tests expect. I also ensured that all temporary files were removed.

[rowan-m]

Do not remove the type hints. The minimum supported PHP version is 8.4.

[google-labs-jules]

Do not remove the type hints. The minimum supported PHP version is 8.4.

I have restored the string type hints for both the argument and the return type in Response::errorJson(). I am keeping the manual string concatenation for now to maintain the exact whitespace formatting required by the existing unit tests.

[rowan-m]

Make sure you address the issues raised by composer run phpstan.
Specifically: Method ReCaptcha\Response::errorJson() should return string but returns string|false.