feat(openai): expose none and xhigh reasoning effort levels

[gargsajal9]

What type of PR is this?

feat

Check the PR title.

• This PR title matches the format: <type>(optional scope): <description>.
• The description is user-oriented and clear enough for others to understand.
• User-facing behavior is documented in the component's English and Chinese READMEs.

(Optional) Translate the PR title into Chinese.

feat(openai): 支持 none 和 xhigh 推理强度级别

(Optional) More detailed description for this PR(en: English/zh: Chinese).

en:

Expose ReasoningEffortLevelNone and ReasoningEffortLevelXHigh from the public components/model/openai package, bringing the wrapper API in line with the reasoning levels supported by OpenAI and ACL main.

The component currently pins released ACL v0.1.17, which does not contain the corresponding named constants. To preserve standalone module compatibility without a dependency upgrade, the new public values are explicitly typed string constants. Existing string pass-through, omission behavior, and provider/model defaults remain unchanged.

This also:

• documents model-dependent support and the difference between explicit none and an omitted value;
• documents per-call overrides through WithReasoningEffort;
• adds external-package HTTP serialization tests for both Generate and Stream, covering new and existing levels, omission, custom values, overrides, and config preservation.

Validation:

• GOWORK=off go test -mod=readonly -race -gcflags="all=-l -N" ./...
• GOWORK=off go vet -mod=readonly ./...
• GOWORK=off go mod tidy -diff
• gofmt and git diff --check

The isolated module validation resolves github.com/cloudwego/eino-ext/libs/acl/openai v0.1.17 with no workspace replacement.

zh(optional):

在公共 components/model/openai 包中新增 ReasoningEffortLevelNone 和 ReasoningEffortLevelXHigh，并补充配置、单次调用覆盖及请求序列化测试。由于当前组件依赖的 ACL v0.1.17 尚未发布对应命名常量，本 PR 使用显式类型化字符串常量，以保持独立模块兼容性且不升级依赖。

(Optional) Which issue(s) this PR fixes:

Fixes #947

(optional) The PR that updates user documentation:

N/A — the component's English and Chinese READMEs are updated in this PR.

[CLAassistant]

All committers have signed the CLA.