add tool name to tool call delta streaming events

[bebiksior]

Adds tool name to ToolCallDelta streaming events so apps can display which tool is being called during streaming

Uses a ToolCallDeltaContent enum with Name(String) and Delta(String) variants since providers send either the name (first event) or argument deltas (subsequent events)

pub enum ToolCallDeltaContent {
    Name(String),
    Delta(String),
}

RawStreamingChoice::ToolCallDelta {
    id: String,
    content: ToolCallDeltaContent,
}

[Sytten]

@joshua-mo-143 FYI @bebiksior here is working for caido, I support this PR. This is a breaking change though so let us know if you prefer to have a nullable name instead but this new enum is IMO better to convey the intent of the delta.

[joshua-mo-143]

lgtm!