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 7fed7130c346:

• Workflow run: https://github.com/braintrustdata/bt/actions/runs/32202114784
• Download all artifacts (GitHub CLI): gh run download 32202114784 --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-aarch64-pc-windows-msvc
• artifacts-build-local-x86_64-pc-windows-msvc
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• artifacts-build-local-aarch64-apple-darwin
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[Stephen Belanger (Qard)]

Still overall LGTM, though CI still seems to be failing, and looks like it needs a rebase.

[Cedric / ViaDézo1er (viadezo1er)]

Rebase done