feat: introduce find_resource! macro that works with Cargo or Bazel (#8879)

To support Bazelification in https://github.com/openai/codex/pull/8875,
this PR introduces a new `find_resource!` macro that we use in place of
our existing logic in tests that looks for resources relative to the
compile-time `CARGO_MANIFEST_DIR` env var.

To make this work, we plan to add the following to all `rust_library()`
and `rust_test()` Bazel rules in the project:

```
rustc_env = {
    "BAZEL_PACKAGE": native.package_name(),
},
```

Our new `find_resource!` macro reads this value via
`option_env!("BAZEL_PACKAGE")` so that the Bazel package _of the code
using `find_resource!`_ is injected into the code expanded from the
macro. (If `find_resource()` were a function, then
`option_env!("BAZEL_PACKAGE")` would always be
`codex-rs/utils/cargo-bin`, which is not what we want.)

Note we only consider the `BAZEL_PACKAGE` value when the `RUNFILES_DIR`
environment variable is set at runtime, indicating that the test is
being run by Bazel. In this case, we have to concatenate the runtime
`RUNFILES_DIR` with the compile-time `BAZEL_PACKAGE` value to build the
path to the resource.

In testing this change, I discovered one funky edge case in
`codex-rs/exec-server/tests/common/lib.rs` where we have to _normalize_
(but not canonicalize!) the result from `find_resource!` because the
path contains a `common/..` component that does not exist on disk when
the test is run under Bazel, so it must be semantically normalized using
the [`path-absolutize`](https://crates.io/crates/path-absolutize) crate
before it is passed to `dotslash fetch`.

Because this new behavior may be non-obvious, this PR also updates
`AGENTS.md` to make humans/Codex aware that this API is preferred.

codex-rs/core/tests/suite/cli_stream.rs

@@ -1,6 +1,7 @@
 use assert_cmd::Command as AssertCommand;
 use codex_core::RolloutRecorder;
 use codex_core::protocol::GitInfo;
+use codex_utils_cargo_bin::find_resource;
 use core_test_support::fs_wait;
 use core_test_support::skip_if_no_network;
 use std::time::Duration;
@@ -12,6 +13,16 @@ use wiremock::ResponseTemplate;
 use wiremock::matchers::method;
 use wiremock::matchers::path;
 
+fn repo_root() -> std::path::PathBuf {
+    #[expect(clippy::expect_used)]
+    find_resource!(".").expect("failed to resolve repo root")
+}
+
+fn cli_responses_fixture() -> std::path::PathBuf {
+    #[expect(clippy::expect_used)]
+    find_resource!("tests/cli_responses_fixture.sse").expect("failed to resolve fixture path")
+}
+
 /// Tests streaming chat completions through the CLI using a mock server.
 /// This test:
 /// 1. Sets up a mock server that simulates OpenAI's chat completions API
@@ -23,6 +34,7 @@ async fn chat_mode_stream_cli() {
     skip_if_no_network!();
 
     let server = MockServer::start().await;
+    let repo_root = repo_root();
     let sse = concat!(
         "data: {\"choices\":[{\"delta\":{\"content\":\"hi\"}}]}\n\n",
         "data: {\"choices\":[{\"delta\":{}}]}\n\n",
@@ -53,7 +65,7 @@ async fn chat_mode_stream_cli() {
         .arg("-c")
         .arg("model_provider=\"mock\"")
         .arg("-C")
-        .arg(env!("CARGO_MANIFEST_DIR"))
+        .arg(&repo_root)
         .arg("hello?");
     cmd.env("CODEX_HOME", home.path())
         .env("OPENAI_API_KEY", "dummy")
@@ -127,6 +139,7 @@ async fn exec_cli_applies_experimental_instructions_file() {
     );
 
     let home = TempDir::new().unwrap();
+    let repo_root = repo_root();
     let bin = codex_utils_cargo_bin::cargo_bin("codex").unwrap();
     let mut cmd = AssertCommand::new(bin);
     cmd.arg("exec")
@@ -140,7 +153,7 @@ async fn exec_cli_applies_experimental_instructions_file() {
             "experimental_instructions_file=\"{custom_path_str}\""
         ))
         .arg("-C")
-        .arg(env!("CARGO_MANIFEST_DIR"))
+        .arg(&repo_root)
         .arg("hello?\n");
     cmd.env("CODEX_HOME", home.path())
         .env("OPENAI_API_KEY", "dummy")
@@ -177,8 +190,8 @@ async fn exec_cli_applies_experimental_instructions_file() {
 async fn responses_api_stream_cli() {
     skip_if_no_network!();
 
-    let fixture =
-        std::path::Path::new(env!("CARGO_MANIFEST_DIR")).join("tests/cli_responses_fixture.sse");
+    let fixture = cli_responses_fixture();
+    let repo_root = repo_root();
 
     let home = TempDir::new().unwrap();
     let bin = codex_utils_cargo_bin::cargo_bin("codex").unwrap();
@@ -186,7 +199,7 @@ async fn responses_api_stream_cli() {
     cmd.arg("exec")
         .arg("--skip-git-repo-check")
         .arg("-C")
-        .arg(env!("CARGO_MANIFEST_DIR"))
+        .arg(&repo_root)
         .arg("hello?");
     cmd.env("CODEX_HOME", home.path())
         .env("OPENAI_API_KEY", "dummy")
@@ -213,8 +226,8 @@ async fn integration_creates_and_checks_session_file() -> anyhow::Result<()> {
     let prompt = format!("echo {marker}");
 
     // 3. Use the same offline SSE fixture as responses_api_stream_cli so the test is hermetic.
-    let fixture =
-        std::path::Path::new(env!("CARGO_MANIFEST_DIR")).join("tests/cli_responses_fixture.sse");
+    let fixture = cli_responses_fixture();
+    let repo_root = repo_root();
 
     // 4. Run the codex CLI and invoke `exec`, which is what records a session.
     let bin = codex_utils_cargo_bin::cargo_bin("codex").unwrap();
@@ -222,7 +235,7 @@ async fn integration_creates_and_checks_session_file() -> anyhow::Result<()> {
     cmd.arg("exec")
         .arg("--skip-git-repo-check")
         .arg("-C")
-        .arg(env!("CARGO_MANIFEST_DIR"))
+        .arg(&repo_root)
         .arg(&prompt);
     cmd.env("CODEX_HOME", home.path())
         .env("OPENAI_API_KEY", "dummy")
@@ -343,7 +356,7 @@ async fn integration_creates_and_checks_session_file() -> anyhow::Result<()> {
     cmd2.arg("exec")
         .arg("--skip-git-repo-check")
         .arg("-C")
-        .arg(env!("CARGO_MANIFEST_DIR"))
+        .arg(&repo_root)
         .arg(&prompt2)
         .arg("resume")
         .arg("--last");