Add fallback text for missing images (#397)

# What?
* When a prompt references an image path that doesn’t exist, replace it
with
  ```[missing image: <path>]``` instead of throwing an ENOENT.
* Adds a few unit tests for input-utils as there weren't any beforehand.

# Why?
Right now if you enter an invalid image path (e.g. it doesn't exist),
codex immediately crashes with a ENOENT error like so:
```
Error: ENOENT: no such file or directory, open 'test.png'
   ...
 {
  errno: -2,
  code: 'ENOENT',
  syscall: 'open',
  path: 'test.png'
}
```
This aborts the entire session. A soft fallback lets the rest of the
input continue.

# How?
Wraps the image encoding + inputItem content pushing in a try-catch. 

This is a minimal patch to avoid completely crashing — future work could
surface a warning to the user when this happens, or something to that
effect.

---------

Co-authored-by: Thibault Sottiaux <tibo@openai.com>

codex-cli/src/utils/input-utils.ts

@@ -2,6 +2,7 @@ import type { ResponseInputItem } from "openai/resources/responses/responses";
 
 import { fileTypeFromBuffer } from "file-type";
 import fs from "fs/promises";
+import path from "path";
 
 export async function createInputItem(
   text: string,
@@ -14,17 +15,24 @@ export async function createInputItem(
   };
 
   for (const filePath of images) {
-    /* eslint-disable no-await-in-loop */
-    const binary = await fs.readFile(filePath);
-    const kind = await fileTypeFromBuffer(binary);
-    /* eslint-enable no-await-in-loop */
-    const encoded = binary.toString("base64");
-    const mime = kind?.mime ?? "application/octet-stream";
-    inputItem.content.push({
-      type: "input_image",
-      detail: "auto",
-      image_url: `data:${mime};base64,${encoded}`,
-    });
+    try {
+      /* eslint-disable no-await-in-loop */
+      const binary = await fs.readFile(filePath);
+      const kind = await fileTypeFromBuffer(binary);
+      /* eslint-enable no-await-in-loop */
+      const encoded = binary.toString("base64");
+      const mime = kind?.mime ?? "application/octet-stream";
+      inputItem.content.push({
+        type: "input_image",
+        detail: "auto",
+        image_url: `data:${mime};base64,${encoded}`,
+      });
+    } catch (err) {
+      inputItem.content.push({
+        type: "input_text",
+        text: `[missing image: ${path.basename(filePath)}]`,
+      });
+    }
   }
 
   return inputItem;