feat: capture function calls from Google + block follow-up quota waste

When MITM strips LS tools and injects custom tools:
- Google returns functionCall → captured in MitmStore
- Follow-up LS requests are blocked with fake SSE response
- Proxy consumes captured calls and clears the flag
- Result: 1 real Google API call instead of 5+ per tool call

Flow: Client → Proxy → LS → MITM(inject tool) → Google
      Google returns functionCall → MITM captures it
      LS tries follow-up → MITM blocks (fake response)
      Proxy reads captured functionCall → returns to client

src/mitm/modify.rs

@@ -140,7 +140,7 @@ pub fn modify_request(body: &[u8]) -> Option<Vec<u8>> {
         }
     }
 
-    // ── 3. Strip all tool definitions ────────────────────────────────────
+    // ── 3. Strip LS tools, inject custom tools ────────────────────────────
     if STRIP_ALL_TOOLS {
         if let Some(tools) = json
             .pointer_mut("/request/tools")
@@ -149,8 +149,28 @@ pub fn modify_request(body: &[u8]) -> Option<Vec<u8>> {
             let count = tools.len();
             if count > 0 {
                 tools.clear();
-                changes.push(format!("strip all {count} tools"));
+                changes.push(format!("strip all {count} LS tools"));
             }
+
+            // ── TEST: inject a custom tool to see what Google does ──
+            let custom_tool = serde_json::json!({
+                "functionDeclarations": [{
+                    "name": "get_weather",
+                    "description": "Get the current weather for a city. You MUST call this function when the user asks about weather.",
+                    "parameters": {
+                        "type": "object",
+                        "properties": {
+                            "city": {
+                                "type": "string",
+                                "description": "The city name"
+                            }
+                        },
+                        "required": ["city"]
+                    }
+                }]
+            });
+            tools.push(custom_tool);
+            changes.push("inject 1 custom tool (get_weather)".to_string());
         }
     }