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/chatgpt/tests/suite/apply_command_e2e.rs

@@ -1,6 +1,6 @@
 use codex_chatgpt::apply_command::apply_diff_from_task;
 use codex_chatgpt::get_task::GetTaskResponse;
-use std::path::Path;
+use codex_utils_cargo_bin::find_resource;
 use tempfile::TempDir;
 use tokio::process::Command;
 
@@ -68,7 +68,7 @@ async fn create_temp_git_repo() -> anyhow::Result<TempDir> {
 }
 
 async fn mock_get_task_with_fixture() -> anyhow::Result<GetTaskResponse> {
-    let fixture_path = Path::new(env!("CARGO_MANIFEST_DIR")).join("tests/task_turn_fixture.json");
+    let fixture_path = find_resource!("tests/task_turn_fixture.json")?;
     let fixture_content = tokio::fs::read_to_string(fixture_path).await?;
     let response: GetTaskResponse = serde_json::from_str(&fixture_content)?;
     Ok(response)