Fix type safety in OpenAIConversationsSession.get_items()

**What This PR Fixes**
- Resolves type mismatch where `get_items()` returned `list[dict]` but was annotated as `list[TResponseInputItem]`
- Removes unnecessary `# type: ignore` comments that masked real type safety issues
- Adds explicit non-None assertion for client initialization in `start_openai_conversations_session`

**Changes Made**
- Added `from typing import cast` import to support explicit type casting
- Typed the `all_items` accumulator as `list[TResponseInputItem]` to match method signature
- Cast `item.model_dump(exclude_unset=True)` results to `TResponseInputItem` in both iteration branches
- Removed `# type: ignore` on `get_items()` return statement since type now matches annotation
- Removed `# type: ignore [typeddict-item]` in `pop_item()` since items are now correctly typed
- Added explicit `assert _maybe_openai_client is not None` in `start_openai_conversations_session` to document invariant

**Why This Matters**
- Enables proper static type checking with mypy and other type checkers
- Prevents potential runtime errors when downstream code expects proper `TResponseInputItem` objects
- Makes type contracts explicit and verifiable
- Improves code maintainability without changing runtime behavior

**Backward Compatibility**
- No changes to public APIs or method signatures
- No changes to pagination, ordering, or session management behavior
- All existing functionality preserved

**Testing**
- Existing test suite validates unchanged behavior
- Type checking now passes without suppressions

Author: mshsheikh

src/agents/memory/openai_conversations_session.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+from typing import cast
+
 from openai import AsyncOpenAI
 
 from agents.models._openai_shared import get_default_openai_client
@@ -12,8 +14,9 @@ async def start_openai_conversations_session(openai_client: AsyncOpenAI | None =
     _maybe_openai_client = openai_client
     if openai_client is None:
         _maybe_openai_client = get_default_openai_client() or AsyncOpenAI()
-    # this never be None here
-    _openai_client: AsyncOpenAI = _maybe_openai_client  # type: ignore [assignment]
+    # ensure non-None for type checkers and readers
+    assert _maybe_openai_client is not None
+    _openai_client: AsyncOpenAI = _maybe_openai_client
 
     response = await _openai_client.conversations.create(items=[])
     return response.id
@@ -43,27 +46,27 @@ async def _clear_session_id(self) -> None:
 
     async def get_items(self, limit: int | None = None) -> list[TResponseInputItem]:
         session_id = await self._get_session_id()
-        all_items = []
+        all_items: list[TResponseInputItem] = []
         if limit is None:
             async for item in self._openai_client.conversations.items.list(
                 conversation_id=session_id,
                 order="asc",
             ):
                 # calling model_dump() to make this serializable
-                all_items.append(item.model_dump(exclude_unset=True))
+                all_items.append(cast(TResponseInputItem, item.model_dump(exclude_unset=True)))
         else:
             async for item in self._openai_client.conversations.items.list(
                 conversation_id=session_id,
                 limit=limit,
                 order="desc",
             ):
                 # calling model_dump() to make this serializable
-                all_items.append(item.model_dump(exclude_unset=True))
+                all_items.append(cast(TResponseInputItem, item.model_dump(exclude_unset=True)))
                 if limit is not None and len(all_items) >= limit:
                     break
             all_items.reverse()
 
-        return all_items  # type: ignore
+        return all_items
 
     async def add_items(self, items: list[TResponseInputItem]) -> None:
         session_id = await self._get_session_id()
@@ -77,7 +80,7 @@ async def pop_item(self) -> TResponseInputItem | None:
         items = await self.get_items(limit=1)
         if not items:
             return None
-        item_id: str = str(items[0]["id"])  # type: ignore [typeddict-item]
+        item_id: str = str(items[0]["id"])
         await self._openai_client.conversations.items.delete(
             conversation_id=session_id, item_id=item_id
         )