chore(apply-patch) scenarios for e2e testing (#7567)

## Summary
This PR introduces an End to End test suite for apply-patch, so we can
easily validate behavior against other implementations as well.

## Testing
- [x] These are tests

codex-rs/apply-patch/tests/suite/scenarios.rs

@@ -0,0 +1,114 @@
+use assert_cmd::prelude::*;
+use pretty_assertions::assert_eq;
+use std::collections::BTreeMap;
+use std::fs;
+use std::path::Path;
+use std::path::PathBuf;
+use std::process::Command;
+use tempfile::tempdir;
+
+#[test]
+fn test_apply_patch_scenarios() -> anyhow::Result<()> {
+    for scenario in fs::read_dir("tests/fixtures/scenarios")? {
+        let scenario = scenario?;
+        let path = scenario.path();
+        if path.is_dir() {
+            run_apply_patch_scenario(&path)?;
+        }
+    }
+    Ok(())
+}
+
+/// Reads a scenario directory, copies the input files to a temporary directory, runs apply-patch,
+/// and asserts that the final state matches the expected state exactly.
+fn run_apply_patch_scenario(dir: &Path) -> anyhow::Result<()> {
+    let tmp = tempdir()?;
+
+    // Copy the input files to the temporary directory
+    let input_dir = dir.join("input");
+    if input_dir.is_dir() {
+        copy_dir_recursive(&input_dir, tmp.path())?;
+    }
+
+    // Read the patch.txt file
+    let patch = fs::read_to_string(dir.join("patch.txt"))?;
+
+    // Run apply_patch in the temporary directory. We intentionally do not assert
+    // on the exit status here; the scenarios are specified purely in terms of
+    // final filesystem state, which we compare below.
+    Command::cargo_bin("apply_patch")?
+        .arg(patch)
+        .current_dir(tmp.path())
+        .output()?;
+
+    // Assert that the final state matches the expected state exactly
+    let expected_dir = dir.join("expected");
+    let expected_snapshot = snapshot_dir(&expected_dir)?;
+    let actual_snapshot = snapshot_dir(tmp.path())?;
+
+    assert_eq!(
+        actual_snapshot,
+        expected_snapshot,
+        "Scenario {} did not match expected final state",
+        dir.display()
+    );
+
+    Ok(())
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+enum Entry {
+    File(Vec<u8>),
+    Dir,
+}
+
+fn snapshot_dir(root: &Path) -> anyhow::Result<BTreeMap<PathBuf, Entry>> {
+    let mut entries = BTreeMap::new();
+    if root.is_dir() {
+        snapshot_dir_recursive(root, root, &mut entries)?;
+    }
+    Ok(entries)
+}
+
+fn snapshot_dir_recursive(
+    base: &Path,
+    dir: &Path,
+    entries: &mut BTreeMap<PathBuf, Entry>,
+) -> anyhow::Result<()> {
+    for entry in fs::read_dir(dir)? {
+        let entry = entry?;
+        let path = entry.path();
+        let Some(stripped) = path.strip_prefix(base).ok() else {
+            continue;
+        };
+        let rel = stripped.to_path_buf();
+        let file_type = entry.file_type()?;
+        if file_type.is_dir() {
+            entries.insert(rel.clone(), Entry::Dir);
+            snapshot_dir_recursive(base, &path, entries)?;
+        } else if file_type.is_file() {
+            let contents = fs::read(&path)?;
+            entries.insert(rel, Entry::File(contents));
+        }
+    }
+    Ok(())
+}
+
+fn copy_dir_recursive(src: &Path, dst: &Path) -> anyhow::Result<()> {
+    for entry in fs::read_dir(src)? {
+        let entry = entry?;
+        let path = entry.path();
+        let file_type = entry.file_type()?;
+        let dest_path = dst.join(entry.file_name());
+        if file_type.is_dir() {
+            fs::create_dir_all(&dest_path)?;
+            copy_dir_recursive(&path, &dest_path)?;
+        } else if file_type.is_file() {
+            if let Some(parent) = dest_path.parent() {
+                fs::create_dir_all(parent)?;
+            }
+            fs::copy(&path, &dest_path)?;
+        }
+    }
+    Ok(())
+}