feat(openai): Add Responses API support

[hggzm]

Summary

Implements OpenAI Responses API support with both stateful and stateless conversation modes.

Changes

• Added OpenAIResponsesModel class implementing IChatModel
• Added StreamableHTTPServerTransport support in MCP plugin
• Updated MCP SDK dependency to ^1.25.0
• Supports automatic function calling with recursive execution

Features

• Stateful mode: Uses previous_response_id for conversation context
• Stateless mode: Sends full history each request
• Full tool/function support with proper error handling

Fixes #2364

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