fix(proxy): improve cache hit rate for Codex/Responses requests

prompt_cache_key was falling back to provider.id when the client did not
supply a session, which collapsed every conversation onto a single key
and defeated upstream prefix caching. Only emit the key when a real
client-provided session/thread identity is available; otherwise let the
upstream use its default matching behaviour.

Additional fixes that affect cache stability:
- Canonicalise (sort) JSON keys in outgoing request bodies and in
  tool_call arguments / tool_result content so semantically identical
  requests produce identical byte sequences for upstream prefix caches.
- Exempt JSON Schema property maps (properties, patternProperties,
  definitions, \$defs) from the underscore-prefix filter so user-defined
  schema keys like _id and _meta survive.
- Add a [CacheTrace] debug log with stable hashes for instructions,
  tools, input and include to help diagnose cache misses.
- Thread session_id into the usage logger for request correlation.

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

@@ -3,7 +3,7 @@
 //! 实现 Anthropic ↔ OpenAI 格式转换，用于 OpenRouter 支持
 //! 参考: anthropic-proxy-rs
 
-use crate::proxy::error::ProxyError;
+use crate::proxy::{error::ProxyError, json_canonical::canonical_json_string};
 use serde_json::{json, Value};
 
 const ANTHROPIC_BILLING_HEADER_PREFIX: &str = "x-anthropic-billing-header:";
@@ -371,7 +371,7 @@ fn convert_message_to_openai(
                         "type": "function",
                         "function": {
                             "name": name,
-                            "arguments": serde_json::to_string(&input).unwrap_or_default()
+                            "arguments": canonical_json_string(&input)
                         }
                     }));
                 }
@@ -384,7 +384,7 @@ fn convert_message_to_openai(
                     let content_val = block.get("content");
                     let content_str = match content_val {
                         Some(Value::String(s)) => s.clone(),
-                        Some(v) => serde_json::to_string(v).unwrap_or_default(),
+                        Some(v) => canonical_json_string(v),
                         None => String::new(),
                     };
                     result.push(json!({