fix: strip $schema and unsupported JSON Schema fields from tool params

Google's Gemini API rejects $schema, additionalProperties, $ref,
$defs, default, examples, and title in tool parameter schemas.
OpenCode/MCP tools include these standard JSON Schema fields.
Now recursively stripped during OpenAI→Gemini tool conversion.

src/mitm/modify.rs

@@ -474,7 +474,8 @@ pub fn openai_tools_to_gemini(tools: &[Value]) -> Vec<Value> {
                 "description": func["description"],
             });
             if let Some(params) = func.get("parameters") {
-                decl["parameters"] = uppercase_types(params.clone());
+                let cleaned = clean_schema_for_gemini(uppercase_types(params.clone()));
+                decl["parameters"] = cleaned;
             }
             Some(decl)
         })
@@ -487,6 +488,41 @@ pub fn openai_tools_to_gemini(tools: &[Value]) -> Vec<Value> {
     vec![serde_json::json!({"functionDeclarations": declarations})]
 }
 
+/// Recursively strip JSON Schema fields that Google's Gemini API doesn't accept.
+/// Known unsupported: $schema, additionalProperties, $ref, $defs, default, examples
+fn clean_schema_for_gemini(mut val: Value) -> Value {
+    const STRIP_KEYS: &[&str] = &[
+        "$schema",
+        "additionalProperties",
+        "$ref",
+        "$defs",
+        "default",
+        "examples",
+        "title",
+    ];
+
+    match &mut val {
+        Value::Object(map) => {
+            for key in STRIP_KEYS {
+                map.remove(*key);
+            }
+            let keys: Vec<String> = map.keys().cloned().collect();
+            for key in keys {
+                if let Some(v) = map.remove(&key) {
+                    map.insert(key, clean_schema_for_gemini(v));
+                }
+            }
+        }
+        Value::Array(arr) => {
+            for v in arr.iter_mut() {
+                *v = clean_schema_for_gemini(std::mem::take(v));
+            }
+        }
+        _ => {}
+    }
+    val
+}
+
 /// Convert OpenAI tool_choice to Gemini toolConfig format.
 ///
 /// OpenAI: "auto" | "required" | "none" | {"type":"function","function":{"name":"X"}}