feat(core): support eval test vars templating

[christso]

Closes #1254

Manual verification

Red on main

Loader behavior

{
  "criteria": "Answers {{question}} correctly",
  "input": [
    {
      "role": "user",
      "content": "Answer clearly: {{question}}"
    }
  ],
  "expected_output": [
    {
      "role": "assistant",
      "content": "{{expected_answer}}"
    }
  ]
}

agentv import promptfoo --dry-run

# Converted from promptfoo config: /tmp/.../promptfooconfig.yaml
name: promptfooconfig
tests:
  - id: capital
    input: "Answer clearly: What is the capital of France?"
    assertions:
      - type: equals
        value: Paris
    metadata:
      promptfoo:
        vars:
          question: What is the capital of France?
        prompt_label: prompt-1
        prompt_source: inline

Green on this branch

Loader behavior

{
  "criteria": "Answers What is the capital of France? correctly",
  "input": [
    {
      "role": "user",
      "content": "Answer clearly: What is the capital of France?"
    }
  ],
  "expected_output": [
    {
      "role": "assistant",
      "content": "Paris"
    }
  ]
}

agentv import promptfoo --dry-run

# Converted from promptfoo config: /tmp/.../promptfooconfig.yaml
name: promptfooconfig
tests:
  - id: capital
    input: "Answer clearly: {{question}}"
    vars:
      question: What is the capital of France?
    assertions:
      - type: equals
        value: Paris
    metadata:
      promptfoo:
        vars:
          question: What is the capital of France?
        prompt_label: prompt-1
        prompt_source: inline

[cloudflare-workers-and-pages]

Deploying agentv with    Cloudflare Pages

Latest commit:	ae13b94
Status:	✅  Deploy successful!
Preview URL:	https://fd8a96b3.agentv.pages.dev
Branch Preview URL:	https://feat-1254-eval-vars.agentv.pages.dev

View logs