fix: do not relay partial tool call events to openai client

[dannykopping]

OpenAI's API sends tool call events like this:

data: ... delta":{"tool_calls":[{"index":0,"id":"call_0TxntkwDB66KH8z4RwNqeWrZ","type":"function","function":{"name":"bmcp_coder_coder_list_workspaces","arguments":""}}]}...
data: ... delta":{"tool_calls":[{"index":0,"function":{"arguments":"{\""}}]}...
data: ... delta":{"tool_calls":[{"index":0,"function":{"arguments":"owner"}}]}...
data: ... delta":{"tool_calls":[{"index":0,"function":{"arguments":"\":\""}}]}...
data: ... delta":{"tool_calls":[{"index":0,"function":{"arguments":"admin"}}]}...
data: ... delta":{"tool_calls":[{"index":0,"function":{"arguments":"\"}"}}]}...

The first event specifies the name - and we know from the name whether it's an injected tool or not. Once all partials have been accumulated we have the full tool call payload, but we must not relay any of these events, including the first one, to the client.