fix: Pause text updates during active tool calls (gemini-2.5-pro bug)

[paflopes]

Originally I found this issue with Gemini 2.5 Pro models in OpenRouter and AI Studio. The problem is in the example below

Most of the times, it breaks the AI for the rest of the conversation, sometimes it doesn't.

The issue is that Gemini returns a tool call in the stream, then it sends the same tool but this time in content:

data: {"id":"gen-1755460677-sseVdUiRiZKHTQpgBrkm","provider":"Google AI Studio","model":"google/gemini-2.5-pro","object":"chat.completion.chunk","created":1755460677,"choices":[{"index":0,"delta":{"role":"assistant","content":null,"tool_calls":[{"index":0,"id":"tool_0_glob","type":"function","function":{"name":"glob","arguments":"{\"pattern\":\"**/*.test.ts\"}"}}]},"finish_reason":"tool_calls","native_finish_reason":null,"logprobs":null}]}
data: {"id":"gen-1755460677-sseVdUiRiZKHTQpgBrkm","provider":"Google AI Studio","model":"google/gemini-2.5-pro","object":"chat.completion.chunk","created":1755460677,"choices":[{"index":0,"delta":{"role":"assistant","content":"call:glob{pattern:","reasoning":null,"reasoning_details":[]},"finish_reason":"stop","native_finish_reason":"STOP","logprobs":null}]}
data: {"id":"gen-1755460677-sseVdUiRiZKHTQpgBrkm","provider":"Google AI Studio","model":"google/gemini-2.5-pro","object":"chat.completion.chunk","created":1755460677,"choices":[{"index":0,"delta":{"role":"assistant","content":""},"finish_reason":null,"native_finish_reason":null,"logprobs":null}],"usage":{"prompt_tokens":13280,"completion_tokens":280,"total_tokens":13560,"prompt_tokens_details":{"cached_tokens":0}}}
data: [DONE]

This commit prevents this by not appending any assistant messages after a tool call is made, until it gets a response.