chore: import upstream snapshot with attribution

examples/config-external-defaulttest/README.md

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+# config-external-defaulttest (External Default Test Configuration)
+
+This example demonstrates how to use the `file://` syntax to load `defaultTest` configuration from an external file.
+
+You can run this example with:
+
+```bash
+npx promptfoo@latest init --example config-external-defaulttest
+cd config-external-defaulttest
+```
+
+## Overview
+
+Instead of defining `defaultTest` inline in your main configuration file, you can reference an external YAML or JSON file. This is useful for:
+
+- Sharing common test configurations across multiple projects
+- Keeping your main config file cleaner and more focused
+- Managing complex test configurations separately
+- Version controlling shared test configurations independently
+
+## Structure
+
+```text
+external-defaulttest/
+├── promptfooconfig.yaml      # Main configuration file
+├── shared/
+│   └── defaultTest.yaml      # Shared default test configuration
+└── README.md                 # This file
+```
+
+## Usage
+
+### Inline defaultTest (traditional approach)
+
+```yaml
+# Traditional inline approach
+defaultTest:
+  assert:
+    - type: cost
+      threshold: 0.01
+    - type: latency
+      threshold: 5000
+  options:
+    provider: openai:o4-mini
+```
+
+### External defaultTest (new approach)
+
+```yaml
+# New external file approach
+defaultTest: file://shared/defaultTest.yaml
+```
+
+The external file (`shared/defaultTest.yaml`) contains the same configuration that would normally be inline.
+
+## Benefits
+
+1. **Reusability**: Share the same defaultTest configuration across multiple prompt configurations
+2. **Maintainability**: Update test defaults in one place
+3. **Organization**: Keep complex test configurations separate from your main config
+4. **Collaboration**: Teams can maintain shared test standards in a central location
+
+## Running the Example
+
+```bash
+cd examples/config-external-defaulttest
+npx promptfoo@latest eval
+```
+
+The external defaultTest configuration will be loaded automatically and applied to all test cases that don't override the defaults.
+
+## Advanced Usage
+
+You can also reference defaultTest files from parent directories or absolute paths:
+
+```yaml
+# Reference from parent directory
+defaultTest: file://../shared-configs/defaultTest.yaml
+
+# Reference from project root
+defaultTest: file://configs/testing/defaultTest.yaml
+```

examples/config-external-defaulttest/promptfooconfig.yaml

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+# yaml-language-server: $schema=https://promptfoo.dev/config-schema.json
+description: External defaultTest configuration example
+
+# Load default test configuration from external file
+defaultTest: file://shared/defaultTest.yaml
+
+prompts:
+  - |
+    You are a helpful assistant that answers questions concisely.
+
+    User: {{question}}
+    Assistant:
+
+providers:
+  - openai:o4-mini
+  - openai:gpt-4.1
+
+tests:
+  # These test cases will inherit all assertions and options from defaultTest
+  - vars:
+      question: What is the capital of France?
+    # Additional assertions specific to this test
+    assert:
+      - type: icontains
+        value: Paris
+
+  - vars:
+      question: Explain photosynthesis in one sentence.
+    assert:
+      - type: contains-any
+        value:
+          - sunlight
+          - plants
+          - energy
+          - carbon dioxide
+
+  - vars:
+      question: What is 2 + 2?
+    assert:
+      - type: icontains
+        value: '4'
+
+  # This test overrides the provider from defaultTest
+  - vars:
+      question: Write a haiku about programming.
+    options:
+      provider: openai:gpt-4.1
+    assert:
+      - type: javascript
+        value: |
+          // Check if output resembles a haiku (3 lines)
+          const lines = output.trim().split('\n').filter(line => line.trim().length > 0);
+          return lines.length === 3 ? 1 : 0;

examples/config-external-defaulttest/shared/defaultTest.yaml

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+# Shared default test configuration
+# This file defines common assertions and options that apply to all test cases
+
+# Default variables that can be overridden by individual tests
+vars:
+  temperature: 0.7
+  max_tokens: 150
+
+# Common assertions for all tests
+assert:
+  # Ensure responses aren't empty
+  - type: not-equals
+    value: ''
+
+  # Check response length
+  - type: javascript
+    value: |
+      // Ensure response is between 1 and 500 characters
+      const length = output.length;
+      return length >= 1 && length <= 500 ? 1 : 0;
+    metric: response_length_check
+
+  # Cost control
+  - type: cost
+    threshold: 0.01 # Max 1 cent per test
+
+# Default options
+options:
+  # Default to using o4-mini for all tests
+  provider: openai:o4-mini
+
+  # Transform variables to add timestamp
+  transformVars: 'Object.assign({}, vars, { timestamp: new Date().toISOString(), testId: Math.random().toString(36).substring(7) })'
+
+# Metadata that gets attached to all tests
+metadata:
+  suite: 'default-test-example'
+  version: '1.0.0'