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_responses.rs

@@ -8,7 +8,7 @@
 //! - system prompt 使用 `instructions` 字段而非 system role message
 //! - usage 字段命名与 Anthropic 一致 (input_tokens/output_tokens)
 
-use crate::proxy::error::ProxyError;
+use crate::proxy::{error::ProxyError, json_canonical::canonical_json_string};
 use serde_json::{json, Value};
 
 pub(crate) fn sanitize_anthropic_tool_use_input(name: &str, input: Value) -> Value {
@@ -441,7 +441,7 @@ fn convert_messages_to_input(messages: &[Value]) -> Result<Vec<Value>, ProxyErro
                                 "type": "function_call",
                                 "call_id": id,
                                 "name": name,
-                                "arguments": serde_json::to_string(&arguments).unwrap_or_default()
+                                "arguments": canonical_json_string(&arguments)
                             }));
                         }
 
@@ -462,7 +462,7 @@ fn convert_messages_to_input(messages: &[Value]) -> Result<Vec<Value>, ProxyErro
                                 .unwrap_or("");
                             let output = match block.get("content") {
                                 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(),
                             };