fix: resolve cross-platform test failures on Windows and Ubuntu

- Add proper cross-platform path handling with Path.resolve()
- Fix file encoding issues by explicitly using UTF-8
- Correct mock paths to use google.generativeai instead of gemini_server.genai
- Create setup.py for proper package installation in CI
- Add conftest.py with Windows asyncio compatibility
- Update CI workflow to install package with pip install -e .
- Add import tests to verify package installation
- Set PYTHONPATH in test environment
- Simplify import mechanism in tests

These changes ensure tests pass consistently across Windows, Ubuntu, and macOS platforms.

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

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

tests/test_gemini_server.py

@@ -14,8 +14,7 @@ parent_dir = Path(__file__).resolve().parent.parent
 if str(parent_dir) not in sys.path:
     sys.path.insert(0, str(parent_dir))
 
-try:
-    from gemini_server import (
+from gemini_server import (
     GeminiChatRequest,
     CodeAnalysisRequest,
     read_file_content,
@@ -25,27 +24,6 @@ try:
     DEVELOPER_SYSTEM_PROMPT,
     DEFAULT_MODEL
 )
-except ImportError as e:
-    # If import fails, try alternative import method
-    import importlib.util
-    spec = importlib.util.spec_from_file_location(
-        "gemini_server", 
-        parent_dir / "gemini_server.py"
-    )
-    gemini_server = importlib.util.module_from_spec(spec)
-    sys.modules["gemini_server"] = gemini_server
-    spec.loader.exec_module(gemini_server)
-    
-    from gemini_server import (
-        GeminiChatRequest,
-        CodeAnalysisRequest,
-        read_file_content,
-        prepare_code_context,
-        handle_list_tools,
-        handle_call_tool,
-        DEVELOPER_SYSTEM_PROMPT,
-        DEFAULT_MODEL
-    )
 
 
 class TestModels: