Assemble sandbox/approval/network prompts dynamically (#8961)

- Add a single builder for developer permissions messaging that accepts
SandboxPolicy and approval policy. This builder now drives the developer
“permissions” message that’s injected at session start and any time
sandbox/approval settings change.
- Trim EnvironmentContext to only include cwd, writable roots, and
shell; removed sandbox/approval/network duplication and adjusted XML
serialization and tests accordingly.

Follow-up: adding a config value to replace the developer permissions
message for custom sandboxes.

codex-rs/core/tests/suite/mod.rs

@@ -42,6 +42,7 @@ mod model_overrides;
 mod model_tools;
 mod models_etag_responses;
 mod otel;
+mod permissions_messages;
 mod prompt_caching;
 mod quota_exceeded;
 mod read_file;