fix(translator): resolve invalid function name errors by sanitizing Claude tool names

This commit centralizes tool name sanitization in SanitizeFunctionName,
applying character compliance, starting character rules, and length limits.
It also fixes a regression in gemini_schema tests and preserves MCP-specific
shortening logic while ensuring compliance.

Co-authored-by: factory-droid[bot] <138933559+factory-droid[bot]@users.noreply.github.com>

internal/util/gemini_schema.go

@@ -390,6 +390,11 @@ func addEmptySchemaPlaceholder(jsonStr string) string {
 
 		// If schema has properties but none are required, add a minimal placeholder.
 		if propsVal.IsObject() && !hasRequiredProperties {
+			// DO NOT add placeholder if it's a top-level schema (parentPath is empty)
+			// or if we've already added a placeholder reason above.
+			if parentPath == "" {
+				continue
+			}
 			placeholderPath := joinPath(propsPath, "_")
 			if !gjson.Get(jsonStr, placeholderPath).Exists() {
 				jsonStr, _ = sjson.Set(jsonStr, placeholderPath+".type", "boolean")