Make local environment optional in EnvironmentManager (#23369)

## Summary
- make `EnvironmentManager` local environment/runtime paths optional
- simplify constructor surface around snapshot materialization
- rename local env accessors to `require_local_environment` /
`try_local_environment`

## Validation
- devbox Bazel build for touched crate surfaces
- `//codex-rs/exec-server:exec-server-unit-tests`
- `//codex-rs/app-server-client:app-server-client-unit-tests`
- filtered touched `//codex-rs/core:core-unit-tests` cases

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

@@ -1738,6 +1738,16 @@ impl ThreadRequestProcessor {
         if command.is_empty() {
             return Err(invalid_request("command must not be empty"));
         }
+        // `thread/shellCommand` is app-server's local-host shell escape hatch,
+        // not the normal turn-selected shell tool path.
+        if self
+            .thread_manager
+            .environment_manager()
+            .try_local_environment()
+            .is_none()
+        {
+            return Err(internal_error("local environment is not configured"));
+        }
 
         let (_, thread) = self.load_thread(&thread_id).await?;
         self.submit_core_op(