refactor: standardize LLM provider constructor pattern

[bug-ops]

Epic: #1977

LLM providers use inconsistent construction patterns:

• ClaudeProvider::new(api_key, model, max_tokens) — 3 args
• OpenAiProvider::new(api_key, base_url, model, max_tokens, embed_model, reasoning) — 6 args
• GeminiProvider::new(api_key, model, max_tokens) — 3 args
• OllamaProvider::new(base_url) — 1 arg

Standardize on builder pattern for providers with 4+ constructor args.

Acceptance Criteria:

• OpenAiProvider uses builder pattern
• No other provider has > 4 positional constructor args
• All tests pass

Complexity: Small | Effort: 1h
Ref: ABS-08