## Why
Turn construction passed resolved environments through several layers
while leaving the environment shell unresolved. As a result,
model-visible environment context could fall back to the session shell
instead of reporting the selected remote environment's shell.
Resolve environment metadata at the turn-context boundary so each turn
carries the shell that belongs to its selected environment. Keep request
validation in app-server, where invalid selections can be returned as
straightforward JSON-RPC errors without coupling core turn construction
to that policy.
## What changed
- resolve environment selections eagerly in
`new_turn_context_from_configuration`
- store the full resolved `Shell` on each `TurnEnvironment`
- simplify the now-redundant resolved-environment constructor plumbing
- keep duplicate and unknown-environment validation as a small
app-server preflight
- add a remote-environment integration test that runs a full
`test_codex` turn and verifies the model-visible environment message
reports `bash`
## Testing
- `cargo check -p codex-core --test all -p codex-app-server`
- `remote_test_env_exposes_bash_shell_to_model` on the Linux
remote-executor harness
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