feat(scenarios): complete scenario evaluation system with JSON Schema validation
- Add JSON Schema validation using Ajv for structured output assertions
- Add extractJsonFromOutput() to parse JSON from various formats (direct, markdown, embedded)
- Add validateJsonSchema() for proper schema validation with error messages
- Implement structured error handling with ScenarioExecutionError class
- Add 8 error categories: COLLECTION_NOT_FOUND, NO_COLLECTION, NO_TOOLS, etc.
- Add comprehensive unit tests for execute.ts (16 tests)
- Expand evaluate.test.ts with JSON Schema validation tests (36 tests total)
- Refactor ExpandedRunDetails.tsx into smaller components for assertions and conversation display