refactor: migrate eval-validator.ts from procedural to Zod-based validation

[christso]

Context

The eval file validation in eval-validator.ts uses procedural if/else logic (~500+ lines). A parallel Zod schema (eval-file.schema.ts) was added in #385 for JSON Schema generation, creating two sources of truth.

Proposal

Refactor eval-validator.ts to use the Zod schema as the single source of truth for both:

1. Runtime validation (Zod .safeParse())
2. JSON Schema generation (zod-to-json-schema)

Benefits

• Single source of truth for eval file structure
• Better error messages from Zod
• Removes ~500 lines of manual validation code
• Type-safe parsing (no type casts)

Considerations

• The current procedural validator supports warnings (not just errors) — Zod only does pass/fail
• Custom evaluator types use .passthrough() which needs careful handling
• Backward-compatible aliases (eval_cases, script, expected_outcome) need Zod transforms
• Extensive test coverage exists in eval-validator.test.ts — migration should preserve all test cases

Scope

• packages/core/src/evaluation/validation/eval-validator.ts → refactor to use Zod
• packages/core/test/evaluation/validation/eval-validator.test.ts → update test setup
• Remove the separate eval-file.schema.ts once validator uses Zod natively