groupware: simplify JMAP integration tests with Stalwart 0.16

* don't run the Stalwart container in recovery mode first, just run it
   in "proper" mode, import the configuration and use it like that,
   which only requires skipping the "destroy" steps

 * add code documentation and logging

 * replace the text templating of the Stalwart snapshot by doing the
   same as the formatting script was doing, but directly in Go, removing
   an additional step to perform when modifying a Stalwart
   configuration to use in the integration tests

pkg/jmap/export_prefixed_writer_test.go

@@ -0,0 +1,62 @@
+package jmap
+
+import (
+	"bytes"
+)
+
+// LogLineWriter captures data chunk by chunk, extracts full lines,
+// and passes them directly to log.Printf.
+type LogLineWriter struct {
+	buf     bytes.Buffer
+	printer func(string)
+	lines   *[]string
+}
+
+// NewLogLineWriter initializes the writer with a specific prefix.
+func NewLogLineWriter(printer func(string), lines *[]string) *LogLineWriter {
+	return &LogLineWriter{printer: printer, lines: lines}
+}
+
+// Write intercepts the byte stream and looks for complete text lines.
+func (w *LogLineWriter) Write(p []byte) (n int, err error) {
+	w.buf.Write(p)
+
+	for {
+		bufferedBytes := w.buf.Bytes()
+		idx := bytes.IndexByte(bufferedBytes, '\n')
+		if idx == -1 {
+			break // Line is incomplete; wait for more data
+		}
+
+		// Slice UP TO the newline (idx), omitting the '\n' itself.
+		// Go's log package handles its own line-endings.
+		line := bufferedBytes[:idx]
+
+		// Emit to log.Printf. Using %s works perfectly with []byte
+		// without forcing an expensive string allocation.
+		s := string(line)
+		w.printer(s)
+		if w.lines != nil {
+			*w.lines = append(*w.lines, s)
+		}
+
+		// Advance the buffer past the processed text AND the '\n' (idx + 1)
+		w.buf.Next(idx + 1)
+	}
+
+	return len(p), nil
+}
+
+// Flush ensures that any lingering text without a trailing newline
+// gets safely pushed out to the log before exit.
+func (w *LogLineWriter) Flush() error {
+	if w.buf.Len() > 0 {
+		s := w.buf.String()
+		w.printer(s)
+		if w.lines != nil {
+			*w.lines = append(*w.lines, s)
+		}
+		w.buf.Reset()
+	}
+	return nil
+}