fix(translator): remove broken type uppercasing in OpenAI Responses-to-Gemini translator

The `ConvertOpenAIResponsesRequestToGemini` function had code that attempted
to uppercase JSON Schema type values (e.g. "string" -> "STRING") for Gemini
compatibility. This broke nullable types because when `type` is a JSON array
like `["string", "null"]`:

1. `gjson.Result.String()` returns the raw JSON text `["string","null"]`
2. `strings.ToUpper()` produces `["STRING","NULL"]`
3. `sjson.Set()` stores it as a JSON **string** `"[\"STRING\",\"NULL\"]"`
   instead of a JSON array
4. The downstream `CleanJSONSchemaForGemini()` / `flattenTypeArrays()`
   cannot detect it (since `IsArray()` returns false on a string)
5. Gemini/Antigravity API rejects it with:
   `400 Invalid value at '...type' (Type), "["STRING","NULL"]"`

This was confirmed and tested with Droid Factory (Antigravity) Gemini models
where Claude Code sends tool schemas with nullable parameters.

The fix removes the uppercasing logic entirely and passes the raw schema
through to `parametersJsonSchema`. This is safe because:
- Antigravity executor already runs `CleanJSONSchemaForGemini()` which
  properly handles type arrays, nullable fields, and all schema cleanup
- Gemini/Vertex executors use `parametersJsonSchema` which accepts raw
  JSON Schema directly (no uppercasing needed)
- The uppercasing code also only iterated top-level properties, missing
  nested schemas entirely

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/translator/gemini/openai/responses/gemini_openai-responses_request.go

@@ -330,22 +330,7 @@ func ConvertOpenAIResponsesRequestToGemini(modelName string, inputRawJSON []byte
 					funcDecl, _ = sjson.Set(funcDecl, "description", desc.String())
 				}
 				if params := tool.Get("parameters"); params.Exists() {
-					// Convert parameter types from OpenAI format to Gemini format
-					cleaned := params.Raw
-					// Convert type values to uppercase for Gemini
-					paramsResult := gjson.Parse(cleaned)
-					if properties := paramsResult.Get("properties"); properties.Exists() {
-						properties.ForEach(func(key, value gjson.Result) bool {
-							if propType := value.Get("type"); propType.Exists() {
-								upperType := strings.ToUpper(propType.String())
-								cleaned, _ = sjson.Set(cleaned, "properties."+key.String()+".type", upperType)
-							}
-							return true
-						})
-					}
-					// Set the overall type to OBJECT
-					cleaned, _ = sjson.Set(cleaned, "type", "OBJECT")
-					funcDecl, _ = sjson.SetRaw(funcDecl, "parametersJsonSchema", cleaned)
+					funcDecl, _ = sjson.SetRaw(funcDecl, "parametersJsonSchema", params.Raw)
 				}
 
 				geminiTools, _ = sjson.SetRaw(geminiTools, "0.functionDeclarations.-1", funcDecl)