fix: JSON marshal errors of parameters with optional structs

[maelick]

I experienced an issue similar to #660 when passing a parameter struct to stdlib slog (using a JSONHandler).

From what I was able to reproduce, the error occurs when calling encoding/json.Marshal on missing optional parameters that that the Opt* struct is used in response types. The problem does not seem to occur if there are no response objects including the optional type (because the Encode/Decode methods are not generated on the Opt* types).

For me the error occurred when using slog with a JSONHandler inside an ogen handler:

logger := slog.New(slog.NewJSONHandler(&strings.Builder{}, &slog.HandlerOptions{}))
[...]
logger.Info("test", "params", params)

Which gives this type of log output:

{"time":"2025-03-17T16:13:56.22672301+01:00","level":"INFO","msg":"test","params":"!ERROR:json: error calling MarshalJSON for type api.OptInt: unexpected end of JSON input"}

This PR includes:

• test to reproduce the problem
  • both with encoding/json and slog
  • test can be run without the fix with maelick/ogen@e772bdb
  • the problem specifically occurs when the optional struct is used both as a param and as part of a return type
  • I had to add an endpoint that also returns each value type (see maelick/ogen@14db889), otherwise the Encode/Decode methods that use go-faster/jx are not generated
• a (partial) fix for JSON Marshal error on generated empty OptString struct #660
  • based on Go 1.24 omitzero tag
  • change limited to parameter structs (i.e. does not affect schema structs)
  • add method GoStructTag on Parameter to return struct field tags:
    • omitempty for slices, maps and non-optional nullable fields
    • omitempty and omitzero for optional nullable fields
    • nothing for other fields
  • call method in parameters.tmpl similar to struct.tmpl

Notes:

• I didn't change the minimum Go version in go.mod
  • not sure whether the new tag is backward compatible and whether Go 1.23 would just ignore it or not
  • edit: tests are failing in the pipeline, I assume because Go 1.23 is used to execute them
• I only partially squashed my branch to keep a working commit with the test but without the fix
  • if needed I can re-squash to have one commit with the changes and one with the generated files
  • edit: did more squashing