Add Python SDK thread.run convenience methods (#15088)

## TL;DR
Add `thread.run(...)` / `async thread.run(...)` convenience methods to
the Python SDK for the common case.

- add `RunInput = Input | str` and `RunResult` with `final_response`,
collected `items`, and optional `usage`
- keep `thread.turn(...)` strict and lower-level for streaming,
steering, interrupting, and raw generated `Turn` access
- update Python SDK docs, quickstart examples, and tests for the sync
and async convenience flows

## Validation
- `python3 -m pytest sdk/python/tests/test_public_api_signatures.py
sdk/python/tests/test_public_api_runtime_behavior.py`
- `python3 -m pytest
sdk/python/tests/test_real_app_server_integration.py -k
'thread_run_convenience or async_thread_run_convenience'` (skipped in
this environment)

---------

Co-authored-by: Codex <noreply@openai.com>

sdk/python/src/codex_app_server/_inputs.py

@@ -0,0 +1,63 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+from .models import JsonObject
+
+
+@dataclass(slots=True)
+class TextInput:
+    text: str
+
+
+@dataclass(slots=True)
+class ImageInput:
+    url: str
+
+
+@dataclass(slots=True)
+class LocalImageInput:
+    path: str
+
+
+@dataclass(slots=True)
+class SkillInput:
+    name: str
+    path: str
+
+
+@dataclass(slots=True)
+class MentionInput:
+    name: str
+    path: str
+
+
+InputItem = TextInput | ImageInput | LocalImageInput | SkillInput | MentionInput
+Input = list[InputItem] | InputItem
+RunInput = Input | str
+
+
+def _to_wire_item(item: InputItem) -> JsonObject:
+    if isinstance(item, TextInput):
+        return {"type": "text", "text": item.text}
+    if isinstance(item, ImageInput):
+        return {"type": "image", "url": item.url}
+    if isinstance(item, LocalImageInput):
+        return {"type": "localImage", "path": item.path}
+    if isinstance(item, SkillInput):
+        return {"type": "skill", "name": item.name, "path": item.path}
+    if isinstance(item, MentionInput):
+        return {"type": "mention", "name": item.name, "path": item.path}
+    raise TypeError(f"unsupported input item: {type(item)!r}")
+
+
+def _to_wire_input(input: Input) -> list[JsonObject]:
+    if isinstance(input, list):
+        return [_to_wire_item(i) for i in input]
+    return [_to_wire_item(input)]
+
+
+def _normalize_run_input(input: RunInput) -> Input:
+    if isinstance(input, str):
+        return TextInput(input)
+    return input