### 🛠 Fixes `KeyError: 'content'` when using `stream=False`
#### 🔍 Problem
When calling the chat API with `stream=False`, the code attempts to
access `msg[-1]["content"]` without verifying if the key exists. This
causes a `KeyError` when the message structure does not contain
`"content"`.
This issue was discussed in
[#4885](https://github.com/infiniflow/ragflow/issues/4885), where we
analyzed the root cause. The error does not occur with `stream=True`, as
the response is processed differently.
#### ✅ Solution
- **Logging Fix:**
- Before accessing `msg[-1]["content"]`, we check if the key exists.
- If it does not exist, a default value (`"[content not available]"`) is
used to prevent errors.
- **Structural Fix in `msg` Construction:**
- Ensured that every message in `msg` contains the `"content"` key, even
if empty.
- This fixes the issue at its root and ensures consistent behavior
between `stream=True` and `stream=False`.
#### 🔄 Impact
- Prevents the `KeyError` without affecting normal application flow.
- Ensures the integrity of the `msg` structure, avoiding future
failures.
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
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