Fix remote eval "Not connected" when parameter description is missing

[colinbennettbrain]

Summary

• Fixes remote eval showing "Not connected" even when server is running and returning 200 OK
• Root cause: Python SDK serialized None as null for missing description field, but frontend Zod schema uses .optional() which rejects null

In py/src/braintrust/parameters.py, the parameters_to_json_schema() function was unconditionally including description and default fields:

result[name] = {
    "type": "prompt",
    "default": schema.get("default"),      # None -> null
    "description": schema.get("description"),  # None -> null
}

The frontend Zod schema in js/dev/types.ts uses:
description: z.string().optional(),  // Accepts undefined, REJECTS null

When null is received, evaluatorDefinitionsSchema.safeParse() fails, causing the "Not connected" status.

Fix

Only include description and default fields when they have actual values.

Test plan

• Added unit tests for parameters_to_json_schema()
• Manual testing: Remote eval connects when parameter has no description