fix: Do not include _ldMeta in returned config. (#668)

The primary purpose of this was to add unit tests, but it ended up with
a bug fix, so that is the PR title.

Author: kinyoklion

packages/sdk/server-ai/__tests__/LDAIClientImpl.test.ts

@@ -0,0 +1,136 @@
+import { LDContext } from '@launchdarkly/js-server-sdk-common';
+
+import { LDGenerationConfig } from '../src/api/config';
+import { LDAIClientImpl } from '../src/LDAIClientImpl';
+import { LDClientMin } from '../src/LDClientMin';
+
+const mockLdClient: jest.Mocked<LDClientMin> = {
+  variation: jest.fn(),
+  track: jest.fn(),
+};
+
+const testContext: LDContext = { kind: 'user', key: 'test-user' };
+
+it('interpolates template variables', () => {
+  const client = new LDAIClientImpl(mockLdClient);
+  const template = 'Hello {{name}}, your score is {{score}}';
+  const variables = { name: 'John', score: 42 };
+
+  const result = client.interpolateTemplate(template, variables);
+  expect(result).toBe('Hello John, your score is 42');
+});
+
+it('handles empty variables in template interpolation', () => {
+  const client = new LDAIClientImpl(mockLdClient);
+  const template = 'Hello {{name}}';
+  const variables = {};
+
+  const result = client.interpolateTemplate(template, variables);
+  expect(result).toBe('Hello ');
+});
+
+it('returns model config with interpolated prompts', async () => {
+  const client = new LDAIClientImpl(mockLdClient);
+  const key = 'test-flag';
+  const defaultValue: LDGenerationConfig = {
+    model: { modelId: 'test', name: 'test-model' },
+    prompt: [],
+  };
+
+  const mockVariation = {
+    model: { modelId: 'example-provider', name: 'imagination' },
+    prompt: [
+      { role: 'system', content: 'Hello {{name}}' },
+      { role: 'user', content: 'Score: {{score}}' },
+    ],
+    _ldMeta: {
+      versionKey: 'v1',
+      enabled: true,
+    },
+  };
+
+  mockLdClient.variation.mockResolvedValue(mockVariation);
+
+  const variables = { name: 'John', score: 42 };
+  const result = await client.modelConfig(key, testContext, defaultValue, variables);
+
+  expect(result).toEqual({
+    config: {
+      model: { modelId: 'example-provider', name: 'imagination' },
+      prompt: [
+        { role: 'system', content: 'Hello John' },
+        { role: 'user', content: 'Score: 42' },
+      ],
+    },
+    tracker: expect.any(Object),
+    enabled: true,
+  });
+});
+
+it('includes context in variables for prompt interpolation', async () => {
+  const client = new LDAIClientImpl(mockLdClient);
+  const key = 'test-flag';
+  const defaultValue: LDGenerationConfig = {
+    model: { modelId: 'test', name: 'test-model' },
+    prompt: [],
+  };
+
+  const mockVariation = {
+    prompt: [{ role: 'system', content: 'User key: {{ldctx.key}}' }],
+    _ldMeta: { versionKey: 'v1', enabled: true },
+  };
+
+  mockLdClient.variation.mockResolvedValue(mockVariation);
+
+  const result = await client.modelConfig(key, testContext, defaultValue);
+
+  expect(result.config.prompt?.[0].content).toBe('User key: test-user');
+});
+
+it('handles missing metadata in variation', async () => {
+  const client = new LDAIClientImpl(mockLdClient);
+  const key = 'test-flag';
+  const defaultValue: LDGenerationConfig = {
+    model: { modelId: 'test', name: 'test-model' },
+    prompt: [],
+  };
+
+  const mockVariation = {
+    model: { modelId: 'example-provider', name: 'imagination' },
+    prompt: [{ role: 'system', content: 'Hello' }],
+  };
+
+  mockLdClient.variation.mockResolvedValue(mockVariation);
+
+  const result = await client.modelConfig(key, testContext, defaultValue);
+
+  expect(result).toEqual({
+    config: {
+      model: { modelId: 'example-provider', name: 'imagination' },
+      prompt: [{ role: 'system', content: 'Hello' }],
+    },
+    tracker: expect.any(Object),
+    enabled: false,
+  });
+});
+
+it('passes the default value to the underlying client', async () => {
+  const client = new LDAIClientImpl(mockLdClient);
+  const key = 'non-existent-flag';
+  const defaultValue: LDGenerationConfig = {
+    model: { modelId: 'default-model', name: 'default' },
+    prompt: [{ role: 'system', content: 'Default prompt' }],
+  };
+
+  mockLdClient.variation.mockResolvedValue(defaultValue);
+
+  const result = await client.modelConfig(key, testContext, defaultValue);
+
+  expect(result).toEqual({
+    config: defaultValue,
+    tracker: expect.any(Object),
+    enabled: false,
+  });
+
+  expect(mockLdClient.variation).toHaveBeenCalledWith(key, testContext, defaultValue);
+});