fix(proxy): reduce unnecessary Copilot premium interaction consumption

- Fix request classification: treat messages containing tool_result as
  agent continuation instead of user-initiated, preventing false premium
  charges on every tool call
- Add subagent detection via __SUBAGENT_MARKER__ and metadata._agent_
  fallback, setting x-interaction-type=conversation-subagent
- Add deterministic x-interaction-id derived from session ID to group
  requests into a single billing interaction
- Add orphan tool_result sanitization to prevent upstream API errors
  that could cause retries and duplicate billing
- Reorder pipeline: classify (on original body) → sanitize → merge →
  warmup, ensuring classification sees raw tool_result semantics
- Enable warmup downgrade by default with gpt-5-mini model
- Enhance session ID extraction priority chain for Copilot cache keys
- Detect infinite whitespace bug in streaming tool call arguments

src-tauri/src/proxy/session.rs

@@ -337,7 +337,7 @@ fn extract_from_metadata(body: &serde_json::Value) -> Option<SessionIdResult> {
 /// 从 user_id 解析 session_id
 ///
 /// 格式: `user_identifier_session_actual_session_id`
-fn parse_session_from_user_id(user_id: &str) -> Option<String> {
+pub(super) fn parse_session_from_user_id(user_id: &str) -> Option<String> {
     // 查找 "_session_" 分隔符
     if let Some(pos) = user_id.find("_session_") {
         let session_id = &user_id[pos + 9..]; // "_session_" 长度为 9