Add Python app-server SDK (#14435)

## TL;DR
Bring the Python app-server SDK from `main-with-prs-13953-and-14232`
onto current `main` as a standalone SDK-only PR.

- adds the new `sdk/python` and `sdk/python-runtime` package trees
- keeps the scope to the SDK payload only, without the unrelated
branch-history or workflow changes from the source branch
- regenerates `sdk/python/src/codex_app_server/generated/v2_all.py`
against current `main` schema so the extracted SDK matches today's
protocol definitions

## Validation
- `PYTHONPATH=sdk/python/src python3 -m pytest sdk/python/tests`

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

sdk/python/src/codex_app_server/retry.py

@@ -0,0 +1,41 @@
+from __future__ import annotations
+
+import random
+import time
+from typing import Callable, TypeVar
+
+from .errors import is_retryable_error
+
+T = TypeVar("T")
+
+
+def retry_on_overload(
+    op: Callable[[], T],
+    *,
+    max_attempts: int = 3,
+    initial_delay_s: float = 0.25,
+    max_delay_s: float = 2.0,
+    jitter_ratio: float = 0.2,
+) -> T:
+    """Retry helper for transient server-overload errors."""
+
+    if max_attempts < 1:
+        raise ValueError("max_attempts must be >= 1")
+
+    delay = initial_delay_s
+    attempt = 0
+    while True:
+        attempt += 1
+        try:
+            return op()
+        except Exception as exc:
+            if attempt >= max_attempts:
+                raise
+            if not is_retryable_error(exc):
+                raise
+
+            jitter = delay * jitter_ratio
+            sleep_for = min(max_delay_s, delay) + random.uniform(-jitter, jitter)
+            if sleep_for > 0:
+                time.sleep(sleep_for)
+            delay = min(max_delay_s, delay * 2)