Add stdio exec-server listener

[starr-openai]

Why

This stack adds configured exec-server environments, including environments reached over stdio. Before client-side stdio transports or config can use that path, the exec-server binary itself needs a first-class stdio listen mode so it can speak the same JSON-RPC protocol over stdin/stdout that it already speaks over websockets.

Stack position: this is PR 1 of 5. It is the server-side transport foundation for the stack.

What Changed

• Accept stdio and stdio:// for codex exec-server --listen.
• Promote the existing stdio JsonRpcConnection helper from test-only code into normal exec-server transport code.
• Add parse coverage for stdio listen URLs while preserving the existing websocket default.

Stack

• 1. This PR: Add stdio exec-server listener #20663 - Add stdio exec-server listener
• 2. Add stdio exec-server client transport #20664 - Add stdio exec-server client transport
• 3. Make environment providers own default selection #20665 - Make environment providers own default selection
• 4. Add CODEX_HOME environments TOML provider #20666 - Add CODEX_HOME environments TOML provider
• 5. Load configured environments from CODEX_HOME #20667 - Load configured environments from CODEX_HOME

Split from original draft: #20508

Validation

Not run locally; this was split out of the original draft stack.