aibridge: add Amp provider

[dannykopping]

RFC

Add support for Sourcegraph Amp

Acceptance criteria:

• amp can run seamlessly using both Anthropic and OpenAI (used by oracle feature) 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