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。

go.sum

@@ -1,4 +1,7 @@
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