[sdk/python] Stop advertising HTTP image URLs (#29464)

## Summary

- use generated image data URLs in the Python SDK examples and notebook
- document HTTP and HTTPS image URLs as deprecated and recommend
`LocalImageInput`
- replace the remote-URL integration test with data-URL coverage

`ImageInput` remains available for data URLs. The SDK does not duplicate
app-server URL validation.

## Testing

- `uv run --frozen --no-sync ruff check --output-format=full .`
- `uv run --frozen --no-sync ruff format --check .`
- full Python SDK test suite with an isolated writable
`CODEX_SQLITE_HOME` (119 passed, 38 skipped)

sdk/python/tests/test_app_server_inputs.py

@@ -1,40 +1,45 @@
 from __future__ import annotations
 
+import base64
+
 from app_server_harness import AppServerHarness
 from app_server_helpers import TINY_PNG_BYTES
 
 from openai_codex import Codex, ImageInput, LocalImageInput, SkillInput, TextInput
 
 
-def test_remote_image_input_reaches_responses_api(
+def test_data_url_image_input_reaches_responses_api(
     tmp_path,
 ) -> None:
-    """Remote image inputs should survive the SDK and app-server boundary."""
-    remote_image_url = "https://example.com/codex.png"
+    """Data URL image inputs should survive the SDK and app-server boundary."""
+    image_data_url = "data:image/png;base64," + base64.b64encode(TINY_PNG_BYTES).decode("ascii")
 
     with AppServerHarness(tmp_path) as harness:
         harness.responses.enqueue_assistant_message(
-            "remote image received",
-            response_id="remote-image",
+            "data URL image received",
+            response_id="data-url-image",
         )
 
         with Codex(config=harness.app_server_config()) as codex:
             result = codex.thread_start().run(
                 [
-                    TextInput("Describe the remote image."),
-                    ImageInput(remote_image_url),
+                    TextInput("Describe the data URL image."),
+                    ImageInput(image_data_url),
                 ]
             )
             request = harness.responses.single_request()
 
     assert {
         "final_response": result.final_response,
-        "contains_user_prompt": "Describe the remote image." in request.message_input_texts("user"),
-        "image_urls": request.message_image_urls("user"),
+        "contains_user_prompt": "Describe the data URL image."
+        in request.message_input_texts("user"),
+        "image_url_is_png_data_url": request.message_image_urls("user")[-1].startswith(
+            "data:image/png;base64,"
+        ),
     } == {
-        "final_response": "remote image received",
+        "final_response": "data URL image received",
         "contains_user_prompt": True,
-        "image_urls": [remote_image_url],
+        "image_url_is_png_data_url": True,
     }