Standardize config classes

[BreyndotEchse]

• Downgrade symfony/cache to 6.4 (LTS): 7.x requires PHP 8.2+
• Move openai-php/client to require-dev & suggest: OpenAI is not required for Ollama or Anthropic chats. Treat it like other soft deps
• Standardize config classes: Config classes were rather different. One used public properties only, another accepted constructor params as well. One used constants, another enums. One set its defaults in the config class, another used a constant in the respective chat class and so on... Now everything concerning configuration is found in the config classes.

[BreyndotEchse]

Error is false-positive. Runs without any issues with latest pestphp/pest*, phpstan/phpstan, rector/re
ctor versions

[fballiano]

Since I only use ollama I'd also prefer to have openai-sdk as optional, while php8.2+ is file for me