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/fixtures/scenarios/README.md

@@ -0,0 +1,18 @@
+# Overview
+This directory is a collection of end to end tests for the apply-patch specification, meant to be easily portable to other languages or platforms.
+
+
+# Specification
+Each test case is one directory, composed of input state (input/), the patch operation (patch.txt), and the expected final state (expected/). This structure is designed to keep tests simple (i.e. test exactly one patch at a time) while still providing enough flexibility to test any given operation across files.
+
+Here's what this would look like for a simple test apply-patch test case to create a new file:
+
+```
+001_add/
+  input/
+    foo.md
+  expected/
+    foo.md
+    bar.md
+  patch.txt
+```