Deduplicate code across providers and core logic

[LarsArtmann]

The dupl tool identified 431 clone groups in the codebase.

Significant duplication was found in:

• Provider implementations (anthropic, google, openai, openrouter, openaicompat)
  • language_model_hooks.go
  • provider_options.go
  • Core provider logic
• agent.go
• Test files (*_test.go, providertests)

We should refactor this code to use shared abstractions where possible to improve maintainability and reduce technical debt.