chore: import upstream snapshot with attribution

examples/provider-custom/embeddings/README.md

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+# provider-custom/embeddings (Custom Provider Embeddings)
+
+You can run this example with:
+
+```bash
+npx promptfoo@latest init --example provider-custom/embeddings
+cd provider-custom/embeddings
+```
+
+## Usage
+
+This example uses a custom API provider in `customProvider.js`.
+
+Run:
+
+```bash
+promptfoo eval
+```

examples/provider-custom/embeddings/customProvider.js

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+class CustomApiProvider {
+  constructor(options) {
+    // The caller may override Provider ID (e.g. when using multiple instances of the same provider)
+    this.providerId = options.id || 'custom provider';
+
+    // The config object contains any options passed to the provider in the config file.
+    this.config = options.config;
+  }
+
+  id() {
+    return this.providerId;
+  }
+
+  async callApi(prompt) {
+    return null;
+  }
+
+  async callEmbeddingApi(prompt) {
+    const body = {
+      model: 'text-embedding-3-large',
+      input: prompt,
+    };
+    const response = await fetch('https://api.openai.com/v1/embeddings', {
+      method: 'POST',
+      headers: {
+        'Content-Type': 'application/json',
+        Authorization: `Bearer ${process.env.OPENAI_API_KEY}`,
+      },
+      body: JSON.stringify(body),
+    });
+
+    const data = await response.json();
+    if (!data) {
+      return {
+        error: 'Unknown error',
+      };
+    }
+    const ret = {
+      embedding: data.data[0].embedding,
+      tokenUsage: {
+        total: data.usage.total_tokens,
+        prompt: data.usage.prompt_tokens,
+        completion: 0,
+      },
+    };
+    return ret;
+  }
+}
+
+module.exports = CustomApiProvider;

examples/provider-custom/embeddings/promptfooconfig.yaml

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+# yaml-language-server: $schema=https://promptfoo.dev/config-schema.json
+description: Custom provider implementation with embedding similarity
+
+prompts:
+  - 'Translate this to French: {{message}}'
+
+providers:
+  - openai:chat:gpt-4.1-mini
+
+tests:
+  - vars:
+      message: Hello world
+    assert:
+      - type: similar
+        threshold: 0.25
+        value: 'Greetings earth'
+        provider: ./customProvider.js