refactor: deduplicate and improve OpenAI Responses API conversion

- Extract shared map_responses_stop_reason and build_anthropic_usage_from_responses into transform_responses.rs as pub(crate)
- Align cache token extraction priority: OpenAI nested details as fallback, direct Anthropic fields as override
- Extract resolve_content_index helper to eliminate 3x copy-paste in streaming_responses.rs
- Add streaming reasoning/thinking event handlers (response.reasoning.delta/done)
- Add explanatory comment to transform_response heuristic detection
- Add openai_responses to api_format doc comment and needs_transform test
- Add explicit no-op match arms for lifecycle events
- Add promptCacheKey to TS ProviderMeta type
- Update toast i18n key to be generic for both OpenAI formats (zh/en/ja)

src-tauri/src/provider.rs

@@ -233,8 +233,14 @@ pub struct ProviderMeta {
     /// Claude API 格式（仅 Claude 供应商使用）
     /// - "anthropic": 原生 Anthropic Messages API，直接透传
     /// - "openai_chat": OpenAI Chat Completions 格式，需要转换
+    /// - "openai_responses": OpenAI Responses API 格式，需要转换
     #[serde(rename = "apiFormat", skip_serializing_if = "Option::is_none")]
     pub api_format: Option<String>,
+    /// Prompt cache key for OpenAI-compatible endpoints.
+    /// When set, injected into converted requests to improve cache hit rate.
+    /// If not set, provider ID is used automatically during format conversion.
+    #[serde(rename = "promptCacheKey", skip_serializing_if = "Option::is_none")]
+    pub prompt_cache_key: Option<String>,
 }
 
 impl ProviderManager {