feat: support mcp_list_tools

[maralbahari]

Stack

• Depends on: fix: Emit OpenAI-compatible SSE event headers #167

Summary

Expose MCP tool discovery through the Responses API using OpenAI-compatible mcp_list_tools output items and streaming lifecycle events.

The gateway previously discovered remote MCP tools internally but did not include the discovery lifecycle in the public response.

Changes

• Add mcp_list_tools as a first-class response output item.
• Use McpClient::list_tools() during MCP handler initialization to discover tools from the remote server.
• Represent each discovered tool with:
  • name
  • description
  • input schema
  • read_only annotation
• Apply allowed_tools filtering to the discovered tool set.
• Retain the discovery output in the request-scoped tool registry.
• Include an mcp_list_tools item in streaming and non-streaming response output.
• Emit the OpenAI-compatible streaming lifecycle:
  • response.output_item.added
  • response.mcp_list_tools.in_progress
  • response.mcp_list_tools.completed
  • response.output_item.done
• Preserve the same mcpl_ item ID and output index throughout the lifecycle.
• Add support for response.mcp_list_tools.failed.
• Extend ResponseAccumulator to reconstruct mcp_list_tools items from upstream streaming events.
• Keep mcp_list_tools as public response metadata rather than an executable tool call or client-action item.

Testing

• Added serialization and round-trip coverage for mcp_list_tools output items.
• Added accumulator coverage for reconstructing list-tools items from output_item.added and output_item.done.
• Added handler and registry coverage for MCP discovery metadata and stable lifecycle identity.
• Added lifecycle assertions for event order, item IDs, output indices, and contiguous sequence numbers.
• Updated MCP integration tests to compare gateway list-tools output and lifecycle events with OpenAI recordings.
• Re-recorded MCP cassettes for:
  • tool discovery without a tool call
  • successful MCP calls
  • missing arguments
  • invalid argument types
  • non-streaming MCP calls
• Allow OpenAI and gateway cassettes to use different MCP server URLs while comparing the remaining MCP declaration and response behavior.