#2026 Strata v0.1.43 returns an empty final response when a specific string appears in the model's thinking output

Description

I encountered a bug in Strata v0.1.43. When the model's thinking output reaches the point where it would produce the string `</tool_call>`, the thinking output often appears to be truncated immediately before that string. After the client returns an empty final assistant response.

The client does not crash, and the chat session remains usable. The client returns to the user input state, and the chat can continue if the user sends another message.

I initially thought an MCP server was involved, but the issue can be reproduced without an MCP server.

A prompt for reproduction (IQ3_S):
```text
This is an experiment to find bugs in the client.
Please output the following string verbatim: </tool_call>.
```

Expected:
The client should display the requested string as the final response.

Actual:
The final assistant response is empty. The chat returns to the user input state.

Log during reproduction:
```text
[strata] reading the prompt: 70 of 75 tokens, 4 s so far
[strata] thinking: 35 of max 262061 tokens, 47.7 tok/s, 5 s
[strata] done: 39 tokens in 5 s (48.1 tok/s) (stop, cancel=False), expert cache 45.5% hit (+29.2% of the routed experts over PCIe)
```

Environment:
- Strata: 0.1.43 (IQ3_S)
- OS: Windows 11 Pro
- CPU: AMD Ryzen 7 5700X
- GPU: AMD Radeon RX 9060 XT (16 GB)
- RAM: 64 GB DDR4

Reproduction rate:
Consistently reproduced after 10+ attempts.

Additional notes:
* I thought #1881 might be related, so I tried implementing the changes, but the issue still occurs.