Increase exec-server environment transport timeouts

[starr-openai]

Why

The environment-backed exec-server transport currently hardcodes 5 second connect and initialize timeouts in client_transport.rs. That is short for SSH-backed stdio environments and remote websocket environments, and there is currently no way to raise those values from CODEX_HOME/environments.toml.

This stacked follow-up raises the default environment transport timeouts and lets each configured environment override them in environments.toml.

What Changed

• raise the default environment transport connect and initialize timeouts from 5s to 10s
• store concrete timeout values on ExecServerTransportParams instead of hardcoding them in connect_for_transport(...)
• add connect_timeout_sec and initialize_timeout_sec to [[environments]] entries in environments.toml
• apply parse-time defaults so runtime transport code receives fully resolved timeout values
• reject connect_timeout_sec on stdio environments because it only applies to websocket transports
• extend parser tests to cover the new fields and defaults

Stack

• base: Make environment provider snapshots path-free #21794
• this PR: configurable environment transport timeouts

Validation

• cd /Users/starr/code/codex-worktrees/exec-env-timeouts-config-20260508/codex-rs && just fmt
• not run: tests