fix: OpenAI provider system_instruction parameter error (Issue #32)

- Fixed OpenAI provider failing with 'unexpected keyword argument system_instruction'
- Added parameter filtering to exclude Gemini-specific params (system_instruction, safety_settings)
- Ensures cross-provider compatibility when using unified CompletionClient
- Added test case to verify the fix works correctly
- Bumped version to 0.4.2

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ebowwa

CHANGELOG.md

@@ -1,5 +1,12 @@
 # Changelog
 
+## [0.4.2] - 2025-08-21
+### Fixed
+- Fixed OpenAI provider error with 'system_instruction' parameter (Issue #32)
+  - OpenAI provider now properly filters out Gemini-specific parameters
+  - Added parameter exclusion for 'system_instruction' and 'safety_settings'
+  - Ensures cross-provider compatibility when using unified CompletionClient
+
 ## [0.4.1] - 2025-08-19
 ### Changed
 - **BREAKING**: Image generation now requires explicit model selection