fix(util): drop null and empty enum values for Gemini schemas

[RsLuna7]

Problem

Gemini rejects a request with enum[n]: cannot be empty (HTTP 400) whenever a tool schema declares an optional enum the JSON Schema way, i.e. with null listed among the allowed values:

{
  "status": {
    "type": ["string", "null"],
    "enum": ["pending", "in_progress", "completed", "cancelled", null]
  }
}

This shape is not exotic — it is what several schema generators emit for an optional enum field (for example Rust's schemars for Option<SomeEnum>), so any client whose tool definitions contain a nullable enum hits the 400 and the whole request fails, not just the affected property.

Cause

convertEnumValuesToStrings in internal/util/gemini_schema.go normalizes every enum entry with gjson.Result.String(). For a JSON null that returns "", so the empty string is written straight back into the enum array and forwarded to Gemini, which only accepts non-empty strings there.

Fix

• Skip null and empty entries while normalizing.
• If nothing valid remains after filtering, delete the enum keyword instead of emitting an empty array, so the property degrades to a plain typed field rather than an invalid empty enum.

convertEnumValuesToStrings runs before addEnumHints, so the hint text generated downstream no longer picks up empty values either — no separate change needed there.

Tests

Adds TestCleanJSONSchemaForAntigravity_RemovesNullAndEmptyEnumValues, built from two schemas that actually triggered the 400 in the field: a nullable todos[].status and a nullable capability_mode. It asserts the filtered enums keep exactly their valid values and that no entry is a non-string or an empty string.

Verification

• go test ./internal/util -count=1 — ok
• go build -o test-output ./cmd/server — ok
• gofmt — clean on both changed files