feat: add region to model type (#1423)

**Requirements**

- [x] I have added test coverage for new or changed functionality
- [x] I have followed the repository's [pull request submission
guidelines](../blob/main/CONTRIBUTING.md#submitting-pull-requests)
- [x] I have validated my changes against all supported platform
versions

**Related issues**


https://launchdarkly.atlassian.net/jira/software/c/projects/AIC/boards/2045?selectedIssue=AIC-2688

**Describe the solution you've provided**

Adds the `region` property to `LDModelConfig`.

**Additional context**

Some model providers, namely Bedrock, require including a region field
on a given call. A call for a sonnet model might look like:
global.anthropic.sonnet-4-7 or us.anthropic.sonnet-4-7 etc.

This field was added on the UI for Bedrock models and is optionally
includable. Users would be expected to prepend their region to their
model names if they're using a provider that requires it.

> The node SDK just passes the model wholesale from the API response, so
no need to do any property passing here

Tested in the Node sdk pointed at a local instance:

<img width="668" height="156" alt="Screenshot 2026-06-05 at 12 26 36 PM"
src="https://github.com/user-attachments/assets/e79a0d19-534f-4a7b-8d47-eae124cbfe1e"
/>

```json
{ name: 'anthropic.claude-opus-4-7', parameters: {}, region: 'us' }
```

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Type-only additive optional field with no runtime behavior changes in
this PR.
> 
> **Overview**
> Adds an optional **`region`** field to **`LDModelConfig`** so AI
Config model objects can carry provider-specific region metadata (e.g.
Bedrock), aligned with the UI and API shape.
> 
> Consumers that pass through the model from the API response get typed
access to `region` without SDK mapping changes.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
3547c34. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: andrewklatzke

packages/sdk/server-ai/src/api/config/types.ts

@@ -37,6 +37,11 @@ export interface LDModelConfig {
    * Additional user-specified parameters.
    */
   custom?: { [index: string]: unknown };
+
+  /**
+   * The region for the model.
+   */
+  region?: string;
 }
 
 export interface LDProviderConfig {