fix(arena): promptarena validate catches unknown assertion types with fuzzy suggestions

[chaholl]

Summary

• promptarena validate now checks assertion types against the eval handler registry
• Unknown types produce clear errors with "did you mean?" suggestions via Levenshtein distance
• Catches common mistakes like substring_present → contains, tool_called → tools_called

Root Cause

Scenario assertion types (turns[].assertions[].type and conversation_assertions[].type) were validated only structurally (JSON Schema accepts any string). The business logic validator didn't cross-reference against the eval handler registry. Invalid types passed validation and silently failed at runtime — the run still reported exit code 0 with errors buried in JSON output.

Fix

• New ValidateAssertionTypes() function in tools/arena/assertions/ iterates all loaded scenarios and checks each assertion type against the eval registry (including aliases)
• Fuzzy matching via Levenshtein distance provides "did you mean?" suggestions for close matches
• Wired into performBusinessLogicValidation() in the validate command, running after config validation

Example output:

❌ Unknown assertion types (2):
  - hero-scenario: conversation_assertions: unknown assertion type "substring_present" (did you mean "contains"?)
  - hero-scenario: turns[0].assertions: unknown assertion type "tool_called" (did you mean "tools_called"?)

Test plan

• TestValidateAssertionTypes/valid_types_produce_no_errors
• TestValidateAssertionTypes/aliases_are_accepted
• TestValidateAssertionTypes/invalid_types_produce_errors_with_suggestions
• TestValidateAssertionTypes/suggestion_includes_close_match
• TestPerformBusinessLogicValidation_InvalidAssertionType — end-to-end with arena config + scenario file
• TestPerformBusinessLogicValidation_ValidAssertionType — no error with valid types

Closes #939

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
94.8% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud