Rename openai_api parameter to openai_api_key with backwards compatibility

[Copilot]

This PR renames the openai_api parameter to openai_api_key across all LLM functions while maintaining full backwards compatibility. The change addresses the issue that openai_api was a non-standard parameter name that was hard to remember.

Key Changes

Parameter Renaming

All LLM functions now accept both openai_api_key (new, preferred) and openai_api (legacy, deprecated) parameters:

• chat_completion()
• extract_fields_from_text()
• match_goals_from_text()
• classify_text()
• synthesize_user_responses()

When both parameters are provided, openai_api_key takes precedence.

Enhanced Configuration Support

Added _get_openai_api_key() helper function that checks configuration in priority order:

1. openai api key: sk-... (highest priority, direct key format)
2. openai: key: sk-... (standardized nested format)
3. open ai: key: sk-... (legacy nested format with space)
4. OPENAI_API_KEY environment variable (lowest priority)

This ensures the openai api key configuration mentioned in the issue becomes the default, highest-priority option.

Documentation Updates

• Updated all function docstrings to reference openai_api_key as the primary parameter
• Added deprecation notes for openai_api parameter
• Updated configuration examples in error messages to use standardized format
• Updated class docstrings to reference new configuration format

Testing

Added comprehensive test suite (test_llms.py) that validates:

• Configuration lookup priority order
• Parameter backwards compatibility
• Edge cases like non-dict config values

Backwards Compatibility

All existing code continues to work unchanged. Users can:

• Continue using openai_api parameter without modification
• Continue using existing configuration formats (open ai: key)
• Gradually migrate to openai_api_key and standardized config formats

Example Usage

# New preferred way
chat_completion(
    system_message="You are a helpful assistant",
    user_message="Hello",
    openai_api_key="sk-..."  # New parameter name
)

# Legacy way (still works)
chat_completion(
    system_message="You are a helpful assistant", 
    user_message="Hello",
    openai_api="sk-..."  # Old parameter name
)

Configuration Examples

# New highest priority format
openai api key: sk-...

# New standardized nested format  
openai:
  key: sk-...
  
# Legacy format (still supported)
open ai:
  key: sk-...

Fixes #278.

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