Tighten exec-server review followups

[starr-openai]

Why

Final review of the stdio/environment stack found two follow-up cleanups. This lands them as a new stacked PR so the approved earlier PRs do not need to be rebased or have CI restarted.

What Changed

• Treat malformed stdio JSON-RPC output as a terminal disconnect, so later calls fail through the existing closed path instead of waiting after the RPC reader exits.
• Add a regression test for the malformed-stdio path rejecting a later call without hanging.
• Keep the provider snapshot/default contract internal by removing the public EnvironmentProvider crate-root re-export and making EnvironmentManager::from_provider crate-private.

Stack

• 1. 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
• 6. This PR: Tighten exec-server review followups

Validation

• git diff --check
• Tests not run locally; this is a follow-up PR intended to let GitHub CI validate without rebasing the existing approved PRs.