json: fix recursive pointer encoding

[felipensp]

Fix #19812

🤖 Generated by Copilot at fff005f

This pull request fixes and tests the JSON encoding and decoding of structs with optional and recursive pointer fields in vlib/v/gen/c/json.v and vlib/json. It adds a type check, removes a variable prefix, and creates a new test file vlib/json/json_encode_recursive_ptr_test.v.

🤖 Generated by Copilot at fff005f

• Fix JSON encoding and decoding of structs with recursive pointer fields (link, link, link)

• Add a test file vlib/json/json_encode_recursive_ptr_test.v that checks the JSON encoding of a struct with a recursive pointer field of type ?&PostTag (link)
• Modify the code generation in vlib/v/gen/c/json.v to handle optional fields with pointer types differently from non-pointer types (link)
• Refactor the code generation in vlib/v/gen/c/json.v to remove the unnecessary res prefix from the variable name that holds the decoded value of the optional field (link)