refactor(httpheader): extract header parsing into dedicated package

Moved HTTP header parsing and building logic from server package to internal/httpheader

internal/httpheader/response.go

@@ -0,0 +1,40 @@
+package httpheader
+
+import (
+	"bytes"
+	"fmt"
+)
+
+func NewResponseHeader(headerData []byte) (ResponseHeader, error) {
+	header := &responseHeader{
+		startLine: nil,
+		headers:   make(map[string]string, 16),
+	}
+
+	lineEnd := bytes.Index(headerData, []byte("\r\n"))
+	if lineEnd == -1 {
+		return nil, fmt.Errorf("invalid response: no CRLF found in start line")
+	}
+
+	header.startLine = headerData[:lineEnd]
+	remaining := headerData[lineEnd+2:]
+	setRemainingHeaders(remaining, header)
+
+	return header, nil
+}
+
+func (resp *responseHeader) Value(key string) string {
+	return resp.headers[key]
+}
+
+func (resp *responseHeader) Set(key string, value string) {
+	resp.headers[key] = value
+}
+
+func (resp *responseHeader) Remove(key string) {
+	delete(resp.headers, key)
+}
+
+func (resp *responseHeader) Finalize() []byte {
+	return finalize(resp.startLine, resp.headers)
+}