MCP tool schema transformation for OpenAI/Azure providers

[ribelo]

Fixes MCP tools failing with OpenAI/Azure providers due to schema compatibility issues.

Resolves:

• #250
• #441

Changes:

• Transform MCP tool schemas to make optional parameters nullable + required for OpenAI/Azure
• Move transformation logic to MCP module for cleaner architecture
• Graceful fallback for complex schemas

Tested with: context7-mcp, works with OpenAI models