[tools] Add apply_patch tool (#2303)

## Summary
We've been seeing a number of issues and reports with our synthetic
`apply_patch` tool, e.g. #802. Let's make this a real tool - in my
anecdotal testing, it's critical for GPT-OSS models, but I'd like to
make it the standard across GPT-5 and codex models as well.

## Testing
- [x] Tested locally
- [x] Integration test

codex-rs/exec/tests/apply_patch.rs

@@ -1,3 +1,5 @@
+#![allow(clippy::expect_used, clippy::unwrap_used)]
+
 use anyhow::Context;
 use assert_cmd::prelude::*;
 use codex_core::CODEX_APPLY_PATCH_ARG1;
@@ -37,3 +39,152 @@ fn test_standalone_exec_cli_can_use_apply_patch() -> anyhow::Result<()> {
     );
     Ok(())
 }
+
+#[cfg(not(target_os = "windows"))]
+#[tokio::test]
+async fn test_apply_patch_tool() -> anyhow::Result<()> {
+    use core_test_support::load_sse_fixture_with_id_from_str;
+    use tempfile::TempDir;
+    use wiremock::Mock;
+    use wiremock::MockServer;
+    use wiremock::ResponseTemplate;
+    use wiremock::matchers::method;
+    use wiremock::matchers::path;
+
+    const SSE_TOOL_CALL_ADD: &str = r#"[
+  {
+    "type": "response.output_item.done",
+    "item": {
+      "type": "function_call",
+      "name": "apply_patch",
+      "arguments": "{\n  \"input\": \"*** Begin Patch\\n*** Add File: test.md\\n+Hello world\\n*** End Patch\"\n}",
+      "call_id": "__ID__"
+    }
+  },
+  {
+    "type": "response.completed",
+    "response": {
+      "id": "__ID__",
+      "usage": {
+        "input_tokens": 0,
+        "input_tokens_details": null,
+        "output_tokens": 0,
+        "output_tokens_details": null,
+        "total_tokens": 0
+      },
+      "output": []
+    }
+  }
+]"#;
+
+    const SSE_TOOL_CALL_UPDATE: &str = r#"[
+  {
+    "type": "response.output_item.done",
+    "item": {
+      "type": "function_call",
+      "name": "apply_patch",
+      "arguments": "{\n  \"input\": \"*** Begin Patch\\n*** Update File: test.md\\n@@\\n-Hello world\\n+Final text\\n*** End Patch\"\n}",
+      "call_id": "__ID__"
+    }
+  },
+  {
+    "type": "response.completed",
+    "response": {
+      "id": "__ID__",
+      "usage": {
+        "input_tokens": 0,
+        "input_tokens_details": null,
+        "output_tokens": 0,
+        "output_tokens_details": null,
+        "total_tokens": 0
+      },
+      "output": []
+    }
+  }
+]"#;
+
+    const SSE_TOOL_CALL_COMPLETED: &str = r#"[
+  {
+    "type": "response.completed",
+    "response": {
+      "id": "__ID__",
+      "usage": {
+        "input_tokens": 0,
+        "input_tokens_details": null,
+        "output_tokens": 0,
+        "output_tokens_details": null,
+        "total_tokens": 0
+      },
+      "output": []
+    }
+  }
+]"#;
+
+    // Start a mock model server
+    let server = MockServer::start().await;
+
+    // First response: model calls apply_patch to create test.md
+    let first = ResponseTemplate::new(200)
+        .insert_header("content-type", "text/event-stream")
+        .set_body_raw(
+            load_sse_fixture_with_id_from_str(SSE_TOOL_CALL_ADD, "call1"),
+            "text/event-stream",
+        );
+
+    Mock::given(method("POST"))
+        // .and(path("/v1/responses"))
+        .respond_with(first)
+        .up_to_n_times(1)
+        .mount(&server)
+        .await;
+
+    // Second response: model calls apply_patch to update test.md
+    let second = ResponseTemplate::new(200)
+        .insert_header("content-type", "text/event-stream")
+        .set_body_raw(
+            load_sse_fixture_with_id_from_str(SSE_TOOL_CALL_UPDATE, "call2"),
+            "text/event-stream",
+        );
+
+    Mock::given(method("POST"))
+        .and(path("/v1/responses"))
+        .respond_with(second)
+        .up_to_n_times(1)
+        .mount(&server)
+        .await;
+
+    let final_completed = ResponseTemplate::new(200)
+        .insert_header("content-type", "text/event-stream")
+        .set_body_raw(
+            load_sse_fixture_with_id_from_str(SSE_TOOL_CALL_COMPLETED, "resp3"),
+            "text/event-stream",
+        );
+
+    Mock::given(method("POST"))
+        // .and(path("/v1/responses"))
+        .respond_with(final_completed)
+        .expect(1)
+        .mount(&server)
+        .await;
+
+    let tmp_cwd = TempDir::new().unwrap();
+    Command::cargo_bin("codex-exec")
+        .context("should find binary for codex-exec")?
+        .current_dir(tmp_cwd.path())
+        .env("CODEX_HOME", tmp_cwd.path())
+        .env("OPENAI_API_KEY", "dummy")
+        .env("OPENAI_BASE_URL", format!("{}/v1", server.uri()))
+        .arg("--skip-git-repo-check")
+        .arg("-s")
+        .arg("workspace-write")
+        .arg("foo")
+        .assert()
+        .success();
+
+    // Verify final file contents
+    let final_path = tmp_cwd.path().join("test.md");
+    let contents = std::fs::read_to_string(&final_path)
+        .unwrap_or_else(|e| panic!("failed reading {}: {e}", final_path.display()));
+    assert_eq!(contents, "Final text\n");
+    Ok(())
+}