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/exec/tests/suite/auth_env.rs

@@ -1,4 +1,5 @@
 #![allow(clippy::unwrap_used, clippy::expect_used)]
+use codex_utils_cargo_bin::find_resource;
 use core_test_support::responses::ev_completed;
 use core_test_support::responses::mount_sse_once_match;
 use core_test_support::responses::sse;
@@ -10,6 +11,7 @@ use wiremock::matchers::header;
 async fn exec_uses_codex_api_key_env_var() -> anyhow::Result<()> {
     let test = test_codex_exec();
     let server = start_mock_server().await;
+    let repo_root = find_resource!(".")?;
 
     mount_sse_once_match(
         &server,
@@ -21,7 +23,7 @@ async fn exec_uses_codex_api_key_env_var() -> anyhow::Result<()> {
     test.cmd_with_server(&server)
         .arg("--skip-git-repo-check")
         .arg("-C")
-        .arg(env!("CARGO_MANIFEST_DIR"))
+        .arg(&repo_root)
         .arg("echo testing codex api key")
         .assert()
         .success();