Keep MCP elicitation routable across runtime refreshes

[jif-oai]

Why

An MCP tool call can still be waiting for an elicitation response when an environment update replaces the thread's MCP runtime.

Before this change:

runtime A starts a tool call and asks the user
environment becomes ready, so runtime B is published
client answers the prompt through runtime B
runtime B cannot find runtime A's pending responder

The response is lost and the original tool call stays blocked.

What changed

All MCP runtimes for one thread now share a small elicitation router:

runtime A ---\
               shared router: response token -> exact pending responder
runtime B ---/

When Codex surfaces an MCP elicitation, it assigns a unique opaque response token. The router records which pending request owns that token. A replacement runtime reuses the same router, so the latest runtime can deliver a response to a request started by the previous runtime.

The Codex-owned token also prevents two runtime connections that reuse the same MCP server request ID from receiving each other's responses.

This does not retain or search old MCP managers. Only the pending responder map is shared.

Covered scenario

The integration test exercises the complete failure mode:

1. A thread starts while its selected environment is still unavailable.
2. A configured MCP server starts a tool call and asks the client for input.
3. The environment becomes ready, causing Codex to publish a replacement MCP runtime.
4. The client answers the original prompt after the replacement.
5. The original tool call receives that answer and completes.

A focused routing test also creates two runtimes with the same server request ID and verifies that each response reaches the exact request that emitted its token.

Scope

This PR changes only elicitation response routing across MCP runtime replacement. It does not change when runtimes are rebuilt, which environments contribute MCP configuration, or how environment availability is detected.

[chatgpt-codex-connector]

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