[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/codex-api/tests/sse_end_to_end.rs

@@ -4,10 +4,10 @@ use anyhow::Result;
 use async_trait::async_trait;
 use bytes::Bytes;
 use codex_api::AuthProvider;
+use codex_api::Compression;
 use codex_api::Provider;
 use codex_api::ResponseEvent;
 use codex_api::ResponsesClient;
-use codex_api::requests::responses::Compression;
 use codex_client::HttpTransport;
 use codex_client::Request;
 use codex_client::Response;
@@ -64,7 +64,7 @@ fn provider(name: &str) -> Provider {
         base_url: "https://example.com/v1".to_string(),
         query_params: None,
         headers: HeaderMap::new(),
-        retry: codex_api::provider::RetryConfig {
+        retry: codex_api::RetryConfig {
             max_attempts: 1,
             base_delay: Duration::from_millis(1),
             retry_429: false,