codex_sdk: route MCP servers into Codex.Thread

[WiggidyW]

Context

The codex_sdk upstream client plumbs MCP server config end-to-end on the Rust side — McpServerConfig { url, headers } is built from the per-agent objectiveai::mcp::Connection and shipped on every run line via RunParams.mcp_servers. The Python runner currently ignores this field; see the NOTE(MCP) comment in objectiveai-codex-sdk-runner-py/main.py::handle_run.

This issue tracks wiring params.mcp_servers into the actual codex Thread so codex agents can call MCP tools.

Why it's deferred

The codex Python SDK's Codex.start_thread / resume_thread and Thread.run_streamed don't expose an MCP-server knob — the SDK's ThreadOptions / TurnOptions types have no field for it. The codex CLI binary itself supports MCP via ~/.codex/config.toml, but that's process-wide config, not per-thread.

Options

1. Per-call config.toml synthesis — write a temporary ~/.codex/config.toml (or --config <path>) snippet before each Thread.run_streamed, scoped to the request's tempdir. Concurrency-hostile: codex reads its config at process start, so this would force a fresh codex subprocess per turn (defeating part of the multiplex win).

2. Wait for SDK support — file an upstream issue / PR on openai-codex-sdk adding an mcp_servers field to ThreadOptions that propagates to the spawned codex exec invocation. Cleanest path; outside our control.

3. Bypass the SDK — invoke the codex binary directly with our own arguments. Loses the SDK's event-streaming wrapper and forces us to re-implement ThreadEvent parsing.

Recommend #2 (waiting / upstream PR) unless we hit a customer with hard MCP requirements; in that case fall back to #1 with a per-request subprocess.

Touchpoints when we do wire it up

• objectiveai-codex-sdk-runner-py/main.py::handle_run — replace the NOTE(MCP) comment with the actual MCP plumbing.
• Optionally relax the _validate_run_params check if mcp_servers becomes required.
• No Rust-side changes expected — RunParams.mcp_servers already serializes the right shape.
• Re-test that codex_sdk::McpServerConfig::from(&Connection) produces the correct headers (Mcp-Session-Id, Authorization, User-Agent, X-Title, Referer/HTTP-Referer) for whatever consumer ends up reading them.

Out of scope

• MCP wiring for codex_sdk agents only. Claude / OpenRouter MCP routing is unchanged.
• The agent-level mcp_servers field on objectiveai::agent::codex_sdk::AgentBase is already declared and validated; this issue is about runtime routing only.