Convert PHP scripts to proper PHPUnit tests and add provider/interface tests for OpenAI

[charvimehradu]

Pull Request for Unit Tests.

Summary of Changes

• Replace many procedural test scripts with proper PHPUnit test classes, organized by provider and interface (Tests/OpenAIProvider).
• Add comprehensive interface tests for OpenAI: ChatInterfaceTest, ModelInterfaceTest, ImageInterfaceTest, AudioInterfaceTest, EmbeddingInterfaceTest, ModerationInterfaceTest, and helpers.
• Add per-test output directory (Tests/OpenAIProvider/output) for optional artifacts produced by tests.

Testing Instructions

Configure provider credentials:

• Create config.json with keys used by tests.

Documentation Changes Required