[sdk-platform-java] Support strict escaping requirements for REST query parameters

Showcase REST integration tests are written to expect the showcase server to throw an `IllegalArgumentException` until the query parameters meet a more strict set of requirements as defined here:

https://github.com/googleapis/gapic-showcase/blob/v0.25.0/util/genrest/resttools/systemparam.go#L37-L46

This issue is referenced in a TODO in the [showcase BUILD.bazel](https://github.com/googleapis/gapic-generator-java/blob/main/showcase/BUILD.bazel#L52-L54) file. To replicate, enable numeric enums and see `DEVELOPMENT.md` for how to run the showcase integration tests.

从 showcase/BUILD.bazel 中的 TODO 开始，并阅读 DEVELOPMENT.md，了解如何运行 showcase 集成测试。将预期的查询参数转义方式与链接的 systemparam.go 部分中的要求进行比较，然后启用数值枚举以复现该失败。当 showcase REST 集成测试通过，且服务器不会针对符合要求的参数抛出 IllegalArgumentException 时，即表示完成。

由索引模型根据 Issue 内容生成。