feat: add JSON/XML/Text/Bytes/Stream/Attachment response helpers

[qm012]

Adds six top-level response helpers that write complete HTTP responses
(headers, status code, body) on top of net/http:

• JSON(w, statusCode, data, opts...) — encodes with encoding/json;
  options: EscapeForHTML(false) to disable HTML escaping (on by default)
  and Indented(true) for pretty-printed output
• XML(w, statusCode, data) — marshals with encoding/xml, writes the
  XML declaration; marshaling errors return before any header is sent
• Text(w, statusCode, s) — text/plain; charset=utf-8 via io.WriteString
• Bytes(w, statusCode, contentType, b) — in-memory raw bytes with a
  caller-supplied content type
• Stream(w, statusCode, contentType, r) — io.Copy from any io.Reader
  with constant memory; takes the sendfile fast path for *os.File.
  Intended for large or not-yet-complete bodies (downloads, proxied
  upstream responses, generated streams); disk files that need Range and
  caching support should use http.ServeFile / http.ServeFileFS
• Attachment(w, filename, r) — sends any io.Reader as a download
  attachment with 200 OK: Content-Disposition is derived from filename
  (Unicode-safe via RFC 5987 through mime.FormatMediaType), Content-Type
  is inferred from the extension with an application/octet-stream
  fallback; built on Stream

Usage

app := sim.NewApp()

app.Get("/user", func(w http.ResponseWriter, r *http.Request) {
    if err := sim.JSON(w, http.StatusOK, user{Name: "gopher", Age: 5}); err != nil {
        slog.Error("write response", "err", err)
    }
})

// File download: Content-Disposition and Content-Type are derived
// from the filename.
app.Get("/download/{file}", func(w http.ResponseWriter, r *http.Request) {
    name := r.PathValue("file")
    f, err := os.Open(name)
    if err != nil {
        sim.JSON(w, http.StatusNotFound, map[string]string{"message": err.Error()})
        return
    }
    defer f.Close()
    sim.Attachment(w, name, f)
})

[qm012]

related #43

[qm012]

/lgtm

[qm012]

/lgtm

[qm012]

/hold

[github-actions]

Merging this branch will increase overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/qm012/sim	91.27% (+1.53%)	👍

---

Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/qm012/sim/responder.go	100.00% (+100.00%)	41 (+41)	41 (+41)	0	🌟

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

Changed unit test files

• github.com/qm012/sim/responder_test.go

[qm012]

/lgtm