[codex] resolve environment shell metadata eagerly (#27709)

## 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

codex-rs/app-server/src/request_processors/request_errors.rs

@@ -1,8 +1,8 @@
 use super::*;
 
-pub(super) fn environment_selection_error_message(err: CodexErr) -> String {
+pub(super) fn environment_selection_error(err: CodexErr) -> JSONRPCErrorError {
     match err {
-        CodexErr::InvalidRequest(message) => message,
-        err => err.to_string(),
+        CodexErr::InvalidRequest(message) => invalid_request(message),
+        err => internal_error(format!("failed to validate environment selections: {err}")),
     }
 }

codex-rs/app-server/src/request_processors/thread_processor.rs

@@ -1299,7 +1299,7 @@ impl ThreadRequestProcessor {
         if let Some(environment_selections) = environment_selections.as_ref() {
             self.thread_manager
                 .validate_environment_selections(environment_selections)
-                .map_err(|err| invalid_request(environment_selection_error_message(err)))?;
+                .map_err(environment_selection_error)?;
         }
         Ok(environment_selections)
     }

codex-rs/app-server/src/request_processors/turn_processor.rs

@@ -346,7 +346,7 @@ impl TurnRequestProcessor {
         if let Some(environment_selections) = environment_selections.as_ref() {
             self.thread_manager
                 .validate_environment_selections(environment_selections)
-                .map_err(|err| invalid_request(environment_selection_error_message(err)))?;
+                .map_err(environment_selection_error)?;
         }
         Ok(environment_selections)
     }