Split cloud config bundle service modules (#25668)

## Summary

- Splits the monolithic `codex-cloud-config` implementation into focused
modules.
- Keeps behavior unchanged from the preceding config bundle runtime
switch.

## Details

This is the reviewability follow-up after the lineage-preserving
migration PRs. The split separates backend transport, loader
construction, cache handling, metrics, validation, service
orchestration, and focused tests into named files.

Verification: `just fmt`; `just test -p codex-cloud-config`.

codex-rs/cloud-config/src/validation.rs

@@ -0,0 +1,34 @@
+use codex_config::AbsolutePathBuf;
+use codex_config::CloudConfigBundle;
+use codex_config::CloudConfigBundleLayers;
+use codex_config::CloudConfigBundleLoadError;
+use codex_config::CloudConfigBundleLoadErrorCode;
+use codex_config::compose_requirements;
+
+pub(crate) fn validate_bundle(
+    bundle: &CloudConfigBundle,
+    base_dir: &AbsolutePathBuf,
+) -> Result<(), CloudConfigBundleLoadError> {
+    let bundle_layers =
+        CloudConfigBundleLayers::from_bundle(bundle.clone(), base_dir).map_err(|err| {
+            CloudConfigBundleLoadError::new(
+                CloudConfigBundleLoadErrorCode::InvalidBundle,
+                /*status_code*/ None,
+                format!("invalid cloud config bundle: {err}"),
+            )
+        })?;
+    let CloudConfigBundleLayers {
+        enterprise_managed_config: _,
+        enterprise_managed_requirements,
+    } = bundle_layers;
+
+    compose_requirements(enterprise_managed_requirements).map_err(|err| {
+        CloudConfigBundleLoadError::new(
+            CloudConfigBundleLoadErrorCode::InvalidBundle,
+            /*status_code*/ None,
+            format!("invalid cloud config bundle: {err}"),
+        )
+    })?;
+
+    Ok(())
+}