[codex] reduce module visibility (#16978)

## Summary
- reduce public module visibility across Rust crates, preferring private
or crate-private modules with explicit crate-root public exports
- update external call sites and tests to use the intended public crate
APIs instead of reaching through module trees
- add the module visibility guideline to AGENTS.md

## Validation
- `cargo check --workspace --all-targets --message-format=short` passed
before the final fix/format pass
- `just fix` completed successfully
- `just fmt` completed successfully
- `git diff --check` passed

codex-rs/backend-client/src/client.rs

@@ -503,6 +503,7 @@ impl Client {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use codex_backend_openapi_models::models::AdditionalRateLimitDetails;
     use pretty_assertions::assert_eq;
 
     #[test]
@@ -536,7 +537,7 @@ mod tests {
                 }))),
                 ..Default::default()
             }))),
-            additional_rate_limits: Some(Some(vec![crate::types::AdditionalRateLimitDetails {
+            additional_rate_limits: Some(Some(vec![AdditionalRateLimitDetails {
                 limit_name: "codex_other".to_string(),
                 metered_feature: "codex_other".to_string(),
                 rate_limit: Some(Some(Box::new(crate::types::RateLimitStatusDetails {
@@ -596,7 +597,7 @@ mod tests {
         let payload = RateLimitStatusPayload {
             plan_type: crate::types::PlanType::Plus,
             rate_limit: None,
-            additional_rate_limits: Some(Some(vec![crate::types::AdditionalRateLimitDetails {
+            additional_rate_limits: Some(Some(vec![AdditionalRateLimitDetails {
                 limit_name: "codex_other".to_string(),
                 metered_feature: "codex_other".to_string(),
                 rate_limit: None,

codex-rs/backend-client/src/lib.rs

@@ -1,5 +1,5 @@
 mod client;
-pub mod types;
+pub(crate) mod types;
 
 pub use client::Client;
 pub use client::RequestError;

codex-rs/backend-client/src/types.rs

@@ -1,4 +1,3 @@
-pub use codex_backend_openapi_models::models::AdditionalRateLimitDetails;
 pub use codex_backend_openapi_models::models::ConfigFileResponse;
 pub use codex_backend_openapi_models::models::CreditStatusDetails;
 pub use codex_backend_openapi_models::models::PaginatedListTaskListItem;