Fix #72: Set default OPENAI_API_KEY to prevent startup crashes

[httpdss]

Summary

Fixes #72 by implementing a graceful solution for missing OPENAI_API_KEY environment variable that prevents application startup crashes.

Changes Made

Core Implementation

• Modified struct_module/model_wrapper.py:
  • Added automatic placeholder API key (sk-default-placeholder-key) when OPENAI_API_KEY is not set for OpenAI models
  • Added warning message when placeholder key is set
  • Enhanced generate_content() method to detect placeholder keys and provide user-friendly messages
  • Improved error handling for API key-related issues

Testing

• Added comprehensive test suite tests/test_model_wrapper.py:
  • Test initialization with existing API key (should not set placeholder)
  • Test initialization without API key (should set placeholder)
  • Test non-OpenAI models (should not interfere)
  • Test content generation with placeholder vs valid keys
  • Test dry run functionality
  • Test enhanced error handling

Benefits

✅ Prevents crashes: Tool starts successfully without OPENAI_API_KEY
✅ Better UX: Users can explore non-AI features immediately
✅ Graceful degradation: Clear messages when AI features are unavailable
✅ Backward compatibility: Existing configurations work unchanged
✅ Smart detection: Only affects OpenAI models, other providers unaffected

Testing Results

• All 42 existing tests pass ✅
• 8 new tests added and passing ✅
• Manual testing confirms no startup crashes ✅
• Warning messages display correctly ✅

Technical Details

The solution implements Option 2 from the issue description with enhanced error handling:

1. Startup Protection: Sets placeholder key only for OpenAI models when OPENAI_API_KEY is missing
2. Clear Communication: Warns users about placeholder usage
3. Smart Fallback: AI generation gracefully fails with helpful messages
4. Minimal Impact: Only affects the specific error case, no changes to working configurations

Acceptance Criteria

• STRUCT tool starts successfully without OPENAI_API_KEY environment variable
• Default/placeholder API key is set programmatically at startup
• Clear warning message when using placeholder API key
• AI features gracefully handle invalid/placeholder API keys
• Existing functionality with valid API keys remains unchanged
• Comprehensive test coverage added

Closes #72

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Codecov Report

❌ Patch coverage is 94.50549% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.74%. Comparing base (4d6a1b1) to head (6fabbc3).

Files with missing lines	Patch %	Lines
tests/test_model_wrapper.py	95.18%	1 Missing and 3 partials ⚠️
struct_module/model_wrapper.py	87.50%	0 Missing and 1 partial ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #74      +/-   ##
==========================================
+ Coverage   66.04%   68.74%   +2.69%     
==========================================
  Files          22       23       +1     
  Lines        1240     1331      +91     
  Branches      175      181       +6     
==========================================
+ Hits          819      915      +96     
+ Misses        384      374      -10     
- Partials       37       42       +5     

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