feat(core): add content preprocessors for llm graders

[christso]

Summary

• add content-file preprocessing support for llm-grader candidate extraction
• support suite-level and per-evaluator preprocessors config with command resolution
• add tests for UTF-8 fallback, command preprocessors, YAML parsing, and llm-grader prompt injection

Status

This is a draft handoff-safe checkpoint for issue #963. The pushed branch passes the repository pre-push checks.

Verification

• bun test packages/core/test/evaluation/content-preprocessor.test.ts packages/core/test/evaluation/preprocessors-yaml.test.ts packages/core/test/evaluation/llm-grader-multimodal.test.ts packages/core/test/evaluation/validation/eval-schema-sync.test.ts
• bun x tsc -p packages/core/tsconfig.json --noEmit
• repository pre-push hooks on git push

Closes #963

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[christso]

Final verification is complete on the latest branch tip 62b4beca.

What changed since the draft checkpoint:

• fixed implicit default llm-grader to inherit suite-level preprocessors
• fixed relative ContentFile.path resolution to use workspace/target base paths
• fixed MIME-vs-alias preprocessor override merging
• added regression coverage for those cases

Verification completed:

• manual CLI e2e via bun apps/cli/src/cli.ts eval ... using a custom file-output target that returned a relative ContentFile path and a custom grader target that only passed when the transformed file text appeared in the grader prompt
• bun test packages/core/test/evaluation/content-preprocessor.test.ts packages/core/test/evaluation/preprocessors-yaml.test.ts packages/core/test/evaluation/orchestrator.test.ts packages/core/test/evaluation/llm-grader-multimodal.test.ts packages/core/test/evaluation/validation/eval-schema-sync.test.ts
• bun x tsc -p packages/core/tsconfig.json --noEmit
• repo pre-push hooks on git push

I’d treat the implementation as ready for review now. The PR is still marked draft only because I haven’t changed the GitHub draft flag via tooling here.

[christso]

Follow-up docs/example update is pushed on top of the review-ready branch.

New commits:

• 294d87fd docs(examples): add content preprocessor example
• d7becdc8 chore(examples): remove generated preprocessor artifact

Added:

• runnable example under examples/features/preprocessors/
• docs updates in evaluators/llm-graders.mdx, evaluation/eval-cases.mdx, and evaluation/examples.mdx
• feature index entry in examples/features/README.md

Verified:

• bun apps/cli/src/cli.ts eval examples/features/preprocessors/evals/dataset.eval.yaml --target file_output
• repo pre-push hooks on git push