### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

`BaseModelResponseIterator._handle_string_chunk` uses a substring check (`if "[DONE]" in str_line`) to detect the SSE termination signal. This causes false stream termination when a provider sends a valid JSON chunk whose string values happen to contain the literal "[DONE]".

For example, if a model generates a tool call with arguments like `{"command": "[DONE] reset"}`, or any JSON payload containing "[DONE]" as a substring, the stream is prematurely cut off. The user sees a truncated response with no error.

This affects all providers using `BaseModelResponseIterator`: Databricks, OpenRouter, OVHCloud, Cloudflare, Triton, A2A, Watsonx, Anthropic completion, Ollama, CometAPI, Bedrock DeepSeek, Vertex AI agent engine.

### Steps to Reproduce

Using any provider that routes through `BaseModelResponseIterator`, trigger a streaming response where the model's output contains the literal string "[DONE]" inside JSON content (e.g. inside tool-call arguments).

The simplest programmatic reproduction:

```python from litellm.llms.base_llm.base_model_ite…