Spring-ai refactor: enhance ChatGenerationMetadata with metadata Map and Builder

• ChatGenerationMetadata provides Map for additional metadata
• Add Builder interface and DefaultChatGenerationMetadataBuilder
• Add DefaultChatGenerationMetadata implementation
• Update all AI model implementations to use the new builder pattern
• Deprecate ChatGenerationMetadata.from() factory method

Resolves #1805

Comment From: markpollack

removed commented out code and made other small changes.

merged in f388f5349a5c92357bea506291640f2703fa9c88