Add runtime assertion authentication modes and an auth provider (#55636)

## What changed

Add `RuntimeAuthProvider` to resolve request authentication through the runtime auth manager. Support `dual_header` mode, which sends a bearer credential alongside `x-openai-agent-assertion`, and `assertion_only` mode, which uses `Authorization: AgentAssertion …`.

Replace stale authentication headers and sign a fresh assertion for each request attempt. Propagate registration failures without falling back to credential-only authentication, preserving policy errors and classifying transport failures, rate limits, and server errors as transient.

Add the disabled-by-default `features.runtime_agent_instances` configuration with independent `enabled` and `mode` settings, schema support, and structured override merging. Selecting a mode alone does not enable the feature.

## Testing

Add coverage for both modes with API key and ChatGPT credentials, stale header replacement, fresh assertions, credential refresh without runtime replacement, registration failures, and configuration parsing and merging.

GitOrigin-RevId: 9e93d25762600f0ff592c5248fe4c8c2c8182c8d

codex-rs/config/src/schema.rs

@@ -153,6 +153,15 @@ pub fn features_schema(schema_gen: &mut SchemaGenerator) -> Schema {
             );
             continue;
         }
+        if feature.id == codex_features::Feature::RuntimeAgentInstances {
+            validation.properties.insert(
+                feature.key.to_string(),
+                schema_gen.subschema_for::<codex_features::FeatureToml<
+                    codex_features::RuntimeAgentInstancesConfigToml,
+                >>(),
+            );
+            continue;
+        }
         if feature.id == codex_features::Feature::AppsMcpPathOverride {
             validation.properties.insert(
                 feature.key.to_string(),