test: add unit tests for refactored create_llm function

  Add comprehensive test coverage for the factory pattern refactoring of
  create_llm and its provider helper functions. Tests cover:

  - All five LLM providers (OpenAI, Azure, Bedrock, Ollama, Anthropic)
  - Validation error handling for missing credentials
  - Optional dependency checks (ImportError scenarios)
  - Optional field handling (base_url, organization, max_tokens, top_p)
  - Common args construction
  - Factory dispatch and provider selection

  Total: 34 test cases in 7 test classes following existing test patterns.

Signed-off-by: Barry H <barraoc@gmail.com>

Author: Barraoc