Return back Minimal ReasonEffort in OpenAIDataModels.kt (#1412)

## Motivation and Context
This change restores the `Minimal ReasonEffort` functionality in
`OpenAIDataModels.kt`, which was previously removed. It ensures
continuity in expected performance and functionality.

## Breaking Changes
No breaking changes are introduced as this restores a previously
existing feature.
---

#### Type of the changes
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update
- [ ] Tests improvement
- [ ] Refactoring
- [ ] CI/CD changes
- [ ] Dependencies update

#### Checklist
- [ ] The pull request has a description of the proposed change
- [ ] I read the [Contributing
Guidelines](https://github.com/JetBrains/koog/blob/main/CONTRIBUTING.md)
before opening the pull request
- [ ] The pull request uses **`develop`** as the base branch
- [ ] Tests for the changes have been added
- [ ] All new and existing tests passed

##### Additional steps for pull requests adding a new feature
- [ ] An issue describing the proposed change exists
- [ ] The pull request includes a link to the issue
- [ ] The change was discussed and approved in the issue
- [ ] Docs have been added / updated

Author: giIiev

prompt/prompt-executor/prompt-executor-clients/prompt-executor-openai-client-base/src/commonMain/kotlin/ai/koog/prompt/executor/clients/openai/base/models/OpenAIDataModels.kt

@@ -478,6 +478,13 @@ public enum class ReasoningEffort {
     @SerialName("none")
     NONE,
 
+    /**
+     * Allows very limited reasoning while strongly prioritizing speed and cost.
+     * Serialized as `"minimal"`.
+     */
+    @SerialName("minimal")
+    MINIMAL,
+
     /**
      * Allows a small amount of reasoning while prioritizing speed and efficiency.
      * Suitable for straightforward prompts with occasional light chaining.