[1/4] Add Python goal routing foundation (#27110)

## Why

Goal continuation turns are emitted by the existing runtime as separate
physical turns. The Python SDK needs private thread-scoped routing
before it can present those notifications as one logical operation,
without changing ordinary turn routing or the app-server protocol.

## What

- add private goal operation state and thread-scoped notification
routing
- add internal wrappers for the existing `thread/goal/clear` and
`thread/goal/set` RPCs
- include existing goal notifications in the SDK notification union
- preserve ordinary turn-ID routing unchanged
- add focused routing coverage

This PR does not expose a public goal API. It is the first PR in the
Python goal operations stack.

## Test plan

- online CI, including the Python SDK suite
- focused typed-notification routing coverage

sdk/python/src/openai_codex/models.py

@@ -27,6 +27,8 @@ from .generated.v2_all import (
     ReasoningSummaryTextDeltaNotification,
     ReasoningTextDeltaNotification,
     TerminalInteractionNotification,
+    ThreadGoalClearedNotification,
+    ThreadGoalUpdatedNotification,
     ThreadNameUpdatedNotification,
     ThreadStartedNotification,
     ThreadTokenUsageUpdatedNotification,
@@ -70,6 +72,8 @@ NotificationPayload: TypeAlias = (
     | ReasoningTextDeltaNotification
     | TerminalInteractionNotification
     | ThreadNameUpdatedNotification
+    | ThreadGoalClearedNotification
+    | ThreadGoalUpdatedNotification
     | ThreadStartedNotification
     | ThreadTokenUsageUpdatedNotification
     | TurnCompletedNotification