aibridge: add Copilot provider

[ssncferreira]

RFC

Add support for Github Copilot

Acceptance criteria:

• copilot can run seamlessly using OpenAI requests (Anthropic requests use the OpenAI request format). These requests are handled by aibridge
  • CLI works as normal during:
    • regular prompts
    • injected MCP tool calls
    • client-side configured MCP
  • VS Code plugin works as normal during:
    • regular prompts
    • injected MCP tool calls
    • client-side configured MCP
• Has tests