feat(scorers): create LLM scorers and classifiers

[Cedric / ViaDézo1er (viadezo1er)]

Add bt scorers create for prompt-based LLM scorers and classifiers.

Parameter validation is a follow-up partly done in #308, and partly done in https://github.com/braintrustdata/braintrust/pull/18155 (and bt will need tweaking once the braintrust pr is merged).
At the moment it is possible to pass incorrect parameters to the scorers, and sometimes the web ui will even show them:

bt scorers create "Demo answer correctness" \
  --slug demo-answer-correctness \
  --description "Synthetic scorer used to demonstrate bt scorers create" \
  --model gpt-5.4-nano \
  --messages @/tmp/bt-scorer-messages.json \
  --choice-scores '{"PASS":1,"FAIL":0}' \
  --use-cot=false \
  --pass-threshold 0.5 \
  --metadata 'purpose: synthetic-demo' \
  --if-exists replace --temperature 1000 --reasoning-effort none

bt-scorer-messages.json:

[
  {
    "role": "system",
    "content": "You are a strict evaluator. Compare the candidate answer with the expected answer. Return exactly PASS or FAIL."
  },
  {
    "role": "user",
    "content": "Question: {{input.question}}\nExpected answer: {{expected}}\nCandidate answer: {{output}}"
  }
]

❯ bt scorers create "Demo answer correctness" \
  --slug demo-answer-correctness \
  --description "Synthetic scorer used to demonstrate bt scorers create" \
  --model gpt-5.4-nano \
  --messages @/tmp/bt-scorer-messages.json \
  --choice-scores '{"PASS":1,"FAIL":0}' \
  --use-cot=false \
  --pass-threshold 0.5 \
  --metadata 'purpose: synthetic-demo' \
  --if-exists replace --json|jq
{
  "found_existing": true,
  "id": "6681c294-0ab3-489b-b979-33351add071d",
  "ignored": false,
  "project_id": "b667bd4c-7e55-49e0-a370-f6526e6bbccd",
  "slug": "demo-answer-correctness",
  "version": "1000197688638740786"
}

[github-actions]

Latest downloadable build artifacts for this PR commit f541a46695f2:

• Workflow run: https://github.com/braintrustdata/bt/actions/runs/31849497706
• Download all artifacts (GitHub CLI): gh run download 31849497706 --repo braintrustdata/bt
• Installers are published from main automatically. To publish one for a PR branch, run release-canary manually via workflow_dispatch.

Available artifact names

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• artifacts-build-local-x86_64-apple-darwin
• artifacts-build-local-aarch64-pc-windows-msvc
• artifacts-build-local-x86_64-pc-windows-msvc
• artifacts-build-local-x86_64-unknown-linux-musl
• artifacts-build-local-x86_64-unknown-linux-gnu
• artifacts-build-local-aarch64-apple-darwin
• artifacts-build-local-aarch64-unknown-linux-gnu
• artifacts-plan-dist-manifest
• cargo-dist-cache

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