feat(tool): Top5#2 完整 JSON-Schema 校验 + 类型强转 + 错误路径 + defaults（对齐 OpenClaw validation）

- 引入 github.com/santhosh-tekuri/jsonschema/v5（纯 Go，OpenClaw/MCP SDK 同思路）。
- validate.go 重写：validateArgs 用 jsonschema Compile+Validate 替换手写子集（required单层），
  支持 required/properties/嵌套 object/array items/additionalProperties/allOf/anyOf/oneOf/
  类型联合/enum/min-max/pattern/defaults；编译器缓存 sync.Map；Draft7。
- coerce.go：对齐 OpenClaw coerceWithJsonSchema 类型强转（string→number/integer/boolean/
  array/object，number/bool→string；union 处理；object/array 递归）。
- defaults.go：applyDefaults 缺失属性按 default 填（含嵌套），先填默认再强转再校验。
- 错误路径：ValidationError.InstanceLocation → 带路径报告（/count: expected integer...）。
- 测试：tool_test.go 更新 required 语义 + 补强转/default/错误路径用例。
go build/vet/test 全绿 + windows/darwin 交叉编译 + gofmt。

internal/runtime/tool/defaults.go

@@ -0,0 +1,81 @@
+package tool
+
+// applyDefaults fills absent properties with their schema "default", recursing
+// into objects and arrays so nested defaults are honoured too.
+func applyDefaults(schema map[string]any, args map[string]any) {
+	props, _ := schema["properties"].(map[string]any)
+	for name, raw := range props {
+		ps, ok := raw.(map[string]any)
+		if !ok {
+			continue
+		}
+		if rawDefault, has := ps["default"]; has {
+			if _, present := args[name]; !present {
+				args[name] = deepCopy(rawDefault)
+			}
+		}
+		if val, present := args[name]; present {
+			applyDefaultsToValue(ps, val)
+		}
+	}
+}
+
+// applyDefaultsToValue recurses into a present argument value to fill nested
+// defaults according to the sub-schema.
+func applyDefaultsToValue(schema map[string]any, val any) {
+	switch v := val.(type) {
+	case map[string]any:
+		applyDefaults(schema, v)
+	case []any:
+		applyDefaultsToArray(schema, v)
+	}
+}
+
+// applyDefaultsToArray fills defaults for array items, handling the schematic,
+// tuple-in-items and 2020-12 prefixItems forms.
+func applyDefaultsToArray(schema map[string]any, v []any) {
+	switch items := schema["items"].(type) {
+	case map[string]any:
+		for _, item := range v {
+			applyDefaultsToValue(items, item)
+		}
+	case []any:
+		for i, item := range v {
+			if i < len(items) {
+				if is, ok := items[i].(map[string]any); ok {
+					applyDefaultsToValue(is, item)
+				}
+			}
+		}
+	}
+	if prefix, ok := schema["prefixItems"].([]any); ok {
+		for i, item := range v {
+			if i < len(prefix) {
+				if is, ok := prefix[i].(map[string]any); ok {
+					applyDefaultsToValue(is, item)
+				}
+			}
+		}
+	}
+}
+
+// deepCopy returns a structurally independent copy of a JSON value so defaults
+// taken from a compiled schema are not corrupted by later in-place coercion.
+func deepCopy(v any) any {
+	switch vv := v.(type) {
+	case map[string]any:
+		out := make(map[string]any, len(vv))
+		for k, val := range vv {
+			out[k] = deepCopy(val)
+		}
+		return out
+	case []any:
+		out := make([]any, len(vv))
+		for i, item := range vv {
+			out[i] = deepCopy(item)
+		}
+		return out
+	default:
+		return vv
+	}
+}