feat: capture thinking text via MITM dual-call merge

The LS makes TWO separate Google API calls for thinking models:
  Call 1: response + thinking token count (no thinking text)
  Call 2: thinking summary text (no thinking tokens)

Each hits a different StreamingAccumulator, so we:
1. Capture response_text in StreamingAccumulator (non-thinking parts)
2. In MitmStore::record_usage, detect when Call 2 arrives for a
   cascade that already has thinking tokens from Call 1
3. Merge Call 2's response_text as thinking_text on Call 1's usage

Also injects includeThoughts into Google API requests via MITM
modify to ensure thinking text is available in SSE responses.

src/mitm/modify.rs

@@ -152,6 +152,47 @@ pub fn modify_request(body: &[u8]) -> Option<Vec<u8>> {
         }
     }
 
+    // ── 4. Inject includeThoughts to capture thinking text ───────────────
+    // Without this flag, Google only reports thinking token counts
+    // but doesn't send the thinking text in SSE parts.
+    {
+        // Ensure request.generationConfig.thinkingConfig.includeThoughts = true
+        let request = json.get_mut("request").and_then(|v| v.as_object_mut());
+        if let Some(req) = request {
+            let gen_config = req
+                .entry("generationConfig")
+                .or_insert_with(|| serde_json::json!({}));
+            if let Some(gc) = gen_config.as_object_mut() {
+                let thinking_config = gc
+                    .entry("thinkingConfig")
+                    .or_insert_with(|| serde_json::json!({}));
+                if let Some(tc) = thinking_config.as_object_mut() {
+                    if !tc.contains_key("includeThoughts") {
+                        tc.insert("includeThoughts".to_string(), Value::Bool(true));
+                        changes.push("inject includeThoughts".to_string());
+                    }
+                }
+            }
+        } else {
+            // Not wrapped in request — try top-level (public API format)
+            let gen_config = json.as_object_mut().and_then(|o| {
+                Some(o.entry("generationConfig")
+                    .or_insert_with(|| serde_json::json!({})))
+            });
+            if let Some(gc) = gen_config.and_then(|v| v.as_object_mut()) {
+                let thinking_config = gc
+                    .entry("thinkingConfig")
+                    .or_insert_with(|| serde_json::json!({}));
+                if let Some(tc) = thinking_config.as_object_mut() {
+                    if !tc.contains_key("includeThoughts") {
+                        tc.insert("includeThoughts".to_string(), Value::Bool(true));
+                        changes.push("inject includeThoughts (top-level)".to_string());
+                    }
+                }
+            }
+        }
+    }
+
     if changes.is_empty() {
         return None; // Nothing modified
     }