[codex] Use AgentAssertion downstream behind use_agent_identity (#17980)

## Summary

This is the AgentAssertion downstream slice for feature-gated agent
identity support, replacing the oversized AgentAssertion slice from PR
#17807.

It isolates task-scoped downstream AgentAssertion wiring on top of the
merged PR3.1 work without re-carrying the earlier agent registration,
task registration, or task-state history.

This PR includes the task-scoped bug-fix call sites from the review:
generic file upload auth, MCP OpenAI file upload auth, and ARC monitor
auth. Broader user/control-plane calls move to PR4.1 and PR4.2.

## Stack

- PR1: https://github.com/openai/codex/pull/17385 - add
`features.use_agent_identity`
- PR2: https://github.com/openai/codex/pull/17386 - register agent
identities when enabled
- PR3: https://github.com/openai/codex/pull/17387 - register agent tasks
when enabled
- PR3.1: https://github.com/openai/codex/pull/17978 - persist and
prewarm registered tasks per thread
- PR4: this PR - use task-scoped `AgentAssertion` downstream when
enabled
- PR4.1: https://github.com/openai/codex/pull/18094 - introduce
AuthManager-owned background/control-plane `AgentAssertion` auth
- PR4.2: https://github.com/openai/codex/pull/18260 - use background
task auth for additional backend/control-plane calls

## What Changed

- add AgentAssertion envelope generation in `codex-core`
- route downstream HTTP and websocket auth through AgentAssertion when
an agent task is present
- extend the model-provider auth provider so non-bearer authorization
schemes can be passed through cleanly
- make generic file uploads attach the full authorization header value
- make MCP OpenAI file uploads use the cached thread agent task
assertion when present
- make ARC monitor calls use the cached thread agent task assertion when
present

## Why

The original PR had drifted ancestry and showed a much larger diff than
the semantic change actually required. Restacking it onto PR3.1 keeps
the reviewable surface down to the downstream assertion slice.

## Validation

- `just fmt`
- `cargo check -p codex-core -p codex-login -p codex-analytics -p
codex-app-server -p codex-cloud-requirements -p codex-cloud-tasks -p
codex-models-manager -p codex-chatgpt -p codex-model-provider -p
codex-mcp -p codex-core-skills`
- `cargo test -p codex-model-provider bearer_auth_provider`
- `cargo test -p codex-core agent_assertion`
- `cargo test -p codex-app-server remote_control`
- `cargo test -p codex-cloud-requirements fetch_cloud_requirements`
- `cargo test -p codex-models-manager manager::tests`
- `cargo test -p codex-chatgpt`
- `cargo test -p codex-cloud-tasks`
- `cargo test -p codex-login agent_identity`
- `just fix -p codex-core -p codex-login -p codex-analytics -p
codex-app-server -p codex-cloud-requirements -p codex-cloud-tasks -p
codex-models-manager -p codex-chatgpt -p codex-model-provider -p
codex-mcp -p codex-core-skills`
- `just fix -p codex-app-server`
- `git diff --check`

codex-rs/codex-api/src/files.rs

@@ -308,6 +308,18 @@ mod tests {
         ChatGptTestAuth
     }
 
+    #[derive(Clone, Copy)]
+    struct AgentAssertionTestAuth;
+
+    impl AuthProvider for AgentAssertionTestAuth {
+        fn add_auth_headers(&self, headers: &mut reqwest::header::HeaderMap) {
+            headers.insert(
+                reqwest::header::AUTHORIZATION,
+                HeaderValue::from_static("AgentAssertion test-assertion"),
+            );
+        }
+    }
+
     fn base_url_for(server: &MockServer) -> String {
         format!("{}/backend-api", server.uri())
     }
@@ -317,6 +329,7 @@ mod tests {
         let server = MockServer::start().await;
         Mock::given(method("POST"))
             .and(path("/backend-api/files"))
+            .and(header("authorization", "Bearer token"))
             .and(header("chatgpt-account-id", "account_id"))
             .and(body_json(serde_json::json!({
                 "file_name": "hello.txt",
@@ -377,4 +390,52 @@ mod tests {
         assert_eq!(uploaded.mime_type, Some("text/plain".to_string()));
         assert_eq!(finalize_attempts.load(Ordering::SeqCst), 2);
     }
+
+    #[tokio::test]
+    async fn upload_local_file_uses_authorization_header_value() {
+        let server = MockServer::start().await;
+        Mock::given(method("POST"))
+            .and(path("/backend-api/files"))
+            .and(header("authorization", "AgentAssertion test-assertion"))
+            .and(body_json(serde_json::json!({
+                "file_name": "hello.txt",
+                "file_size": 5,
+                "use_case": "codex",
+            })))
+            .respond_with(
+                ResponseTemplate::new(200)
+                    .set_body_json(serde_json::json!({"file_id": "file_123", "upload_url": format!("{}/upload/file_123", server.uri())})),
+            )
+            .mount(&server)
+            .await;
+        Mock::given(method("PUT"))
+            .and(path("/upload/file_123"))
+            .and(header("content-length", "5"))
+            .respond_with(ResponseTemplate::new(200))
+            .mount(&server)
+            .await;
+        Mock::given(method("POST"))
+            .and(path("/backend-api/files/file_123/uploaded"))
+            .and(header("authorization", "AgentAssertion test-assertion"))
+            .respond_with(ResponseTemplate::new(200).set_body_json(serde_json::json!({
+                "status": "success",
+                "download_url": format!("{}/download/file_123", server.uri()),
+                "file_name": "hello.txt",
+                "mime_type": "text/plain",
+                "file_size_bytes": 5
+            })))
+            .mount(&server)
+            .await;
+
+        let base_url = base_url_for(&server);
+        let dir = TempDir::new().expect("temp dir");
+        let path = dir.path().join("hello.txt");
+        tokio::fs::write(&path, b"hello").await.expect("write file");
+
+        let uploaded = upload_local_file(&base_url, &AgentAssertionTestAuth, &path)
+            .await
+            .expect("upload succeeds");
+
+        assert_eq!(uploaded.file_id, "file_123");
+    }
 }