Closes#31491
### Problem
Mistral API enforces strict conversation flow requirements that other
providers don't. Specifically, after a `tool` message, the next message
**must** be from the `assistant` role, not `user`. This causes the
error:
```
"Unexpected role 'user' after role 'tool'"
```
This can also occur in normal conversation flow where mistral doesn't
return the assistant message but that is something which can't be
reproduce reliably.
### Root Cause
When users interrupt an ongoing tool call sequence by sending a new
message, we insert a `user` message directly after a `tool` message,
violating Mistral's protocol.
**Expected Mistral flow:**
```
user → assistant (with tool_calls) → tool (results) → assistant (processes results) → user (next input)
```
**What we were doing:**
```
user → assistant (with tool_calls) → tool (results) → user (interruption) ❌
```
### Solution
Insert an empty `assistant` message between any `tool` → `user` sequence
in the Mistral provider's request construction. This satisfies Mistral's
API requirements without affecting other providers or requiring UX
changes.
### Testing
To reproduce the original error:
1. Start agent chat with `codestral-latest`
2. Send: "Describe this project using tool call only"
3. Once tool calls begin, send: "stop this"
4. Main branch: API error
5. This fix: Works correctly
Release Notes:
- Fixed Mistral tool calling in some cases
b8c1b54f9e