fix: tool call streaming

[think-in-universe]

Fixes #150

[Copilot]

Pull Request Overview

This PR introduces specialized delta types for streaming chat completions to properly handle incremental tool call responses. The change addresses the mismatch between complete ToolCall structures and the partial delta chunks received in streaming responses.

• Changed ChatDelta.tool_calls from Vec<ToolCall> to Vec<ToolCallDelta>
• Added new ToolCallDelta struct with optional fields and an index field for chunk ordering
• Added new FunctionCallDelta struct with optional fields for partial function call data

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[hanakannzashi]

use r#type maybe better than type_, this field also exists in some other structs

[think-in-universe]

We can change this all together in another PR. I prefer to keep it consistent for now.

[Copilot]

Pull Request Overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (1)

crates/api/src/conversions.rs:93

• The new ToolCalls finish reason lacks test coverage. The existing tests in this file only cover message and request conversions. Consider adding a test case for finish_reason_to_string that verifies all finish reason variants, including the new ToolCalls variant.

fn finish_reason_to_string(reason: &services::FinishReason) -> String {

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[think-in-universe]

@PierreLeGuen for some context, we have seen below errors when testing the tool call:

Failed to parse chat chunk: missing field `name` for json: {"choices":[{"delta":{"tool_calls":[{"function":{"arguments":"\"}"},"index":0}]},"finish_reason":null,"index":0,"logprobs":null,"token_ids":null}],"created":1762403773,"id":"chatcmpl-746463feebbf402086a89d59d66112ad","model":"deepseek-ai/DeepSeek-V3.1","object":"chat.completion.chunk"}

The name is required in FunctionCall:

/// Function call details
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct FunctionCall {
    pub name: String,
    pub arguments: Option<String>,
}

Also, tool_calls type is needed for FinishReason:

#[derive(Debug, Clone, Serialize, Deserialize)]
#[serde(rename_all = "snake_case")]
pub enum FinishReason {
    Stop,
    Length,
    ContentFilter,
    #[serde(alias = "function_call")]
    ToolCalls,
}