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/core/src/session/session.rs

@@ -1050,14 +1050,13 @@ impl Session {
             .cloned();
             let mcp_runtime_environment = match turn_environment {
                 Some(turn_environment) => McpRuntimeEnvironment::new(
-                    Arc::clone(&turn_environment.environment),
+                    Some(Arc::clone(&turn_environment.environment)),
+                    sess.services.environment_manager.try_local_environment(),
                     turn_environment.cwd.to_path_buf(),
                 ),
                 None => McpRuntimeEnvironment::new(
-                    sess.services
-                        .environment_manager
-                        .default_environment()
-                        .unwrap_or_else(|| sess.services.environment_manager.local_environment()),
+                    sess.services.environment_manager.default_or_local_environment(),
+                    sess.services.environment_manager.try_local_environment(),
                     session_configuration.cwd.to_path_buf(),
                 ),
             };