feat(a2a): add ACP session management and token usage tracking to A2A server

Feature request

Problem

The Codex A2A server currently lacks support for ACP (Agent Client Protocol) session management and token usage tracking. ACP clients have no visibility into token consumption during Codex sessions.

Proposed solution

• Add ACP session lifecycle management to the A2A server
• Integrate token usage tracking via bespoke event handling
• Enhance the MCP server A2A handler with unified tool discovery
• Add session-aware HTTP transport routing

Use case

Enables Codex to work as an ACP-compatible agent, supporting real-time token usage monitoring and multi-turn session management.

Additional context

I have a working implementation with changes across 8 files in codex-rs/a2a and codex-rs/mcp-server. All changes pass cargo check and are backward-compatible.