fix(proxy): correct usage accounting on format-conversion paths

Audited all proxy format-conversion paths (Chat<->Message, Chat<->Response,
Gemini<->Message) for usage/cache metering. Five issues found and fixed.
The dedup mechanism (request_id PK, proxy/session source isolation) is
untouched, so no double-counting is introduced.

- A (Claude + openai_chat, streaming): inject stream_options.include_usage
  so OpenAI-compatible upstreams emit usage in the SSE tail. Without it the
  converted Anthropic message_delta was all-zero and the whole request's
  input/output/cache was dropped. Same root cause as the already-fixed
  Codex Chat path; the injection is extracted into a shared helper
  (transform::inject_openai_stream_include_usage) reused by both paths.

- C (Claude + gemini_native): subtract cachedContentTokenCount from
  input_tokens in build_anthropic_usage so input becomes fresh input
  (Anthropic semantics). Previously the cache-hit tokens were billed twice
  because this path meters as app_type="claude" (input_includes_cache_read
  = false) while Gemini's promptTokenCount includes the cache.

- D (Codex + openai_chat, streaming): gate log_usage on
  has_billable_tokens() to skip the synthetic all-zero usage the converter
  emits when a non-compliant upstream omits usage, preventing empty-row
  request-count inflation.

- P2 (from_claude_stream_events): use has_billable_tokens() for the return
  gate instead of input>0||output>0, so a fully-cached streamed request
  (cache_read>0, input==output==0) is still recorded. Affects all
  Claude-streaming paths, not just Gemini.

- P3 (Codex Chat->Responses, non-streaming): apply the same
  has_billable_tokens() filter the streaming branch got, since the
  synthesized all-zero usage makes from_codex_response return Some and
  bypass the `if let Some` guard.

Add TokenUsage::has_billable_tokens() as the unified predicate. New tests
cover include_usage injection, gemini input subtraction, the gate itself,
cache-only stream recording, and synthetic all-zero codex usage.
Full lib suite: 1569 passed.

src-tauri/src/proxy/providers/transform_codex_chat.rs

@@ -336,21 +336,8 @@ pub fn responses_to_chat_completions_with_reasoning(
     // include_usage 才会在末尾吐 usage chunk。Codex CLI 用 Responses 协议、
     // 自身不带 stream_options，缺这一注入会导致 kimi/MiniMax 等第三方流式请求的
     // token/成本/缓存命中率全部漏记（input/output/cache 全为 0）。
-    let is_stream = result
-        .get("stream")
-        .and_then(|v| v.as_bool())
-        .unwrap_or(false);
-    if is_stream {
-        match result.get_mut("stream_options") {
-            // 保留客户端可能透传的其它 stream_options 字段，仅补 include_usage。
-            Some(Value::Object(opts)) => {
-                opts.insert("include_usage".to_string(), json!(true));
-            }
-            _ => {
-                result["stream_options"] = json!({ "include_usage": true });
-            }
-        }
-    }
+    // 与 Claude→openai_chat 路径共用同一 helper，保证两个客户端方向一致。
+    super::transform::inject_openai_stream_include_usage(&mut result);
 
     Ok(result)
 }