Add full test suite with unit, integration, and E2E smoke tests.

Introduce test.bat orchestrating Go/Vitest/Playwright phases, expand coverage across server, agent, and dashboard, and document in tests/README.md.

server/internal/builder/fusion_upload_limit_test.go

@@ -0,0 +1,43 @@
+package builder
+
+import (
+	"bytes"
+	"mime/multipart"
+	"testing"
+)
+
+type mockMultipartFile struct {
+	*bytes.Reader
+}
+
+func (m *mockMultipartFile) Close() error { return nil }
+
+func newMockFile(data []byte) *mockMultipartFile {
+	return &mockMultipartFile{Reader: bytes.NewReader(data)}
+}
+
+func TestSaveUploadedFusionPayloadRejectsDeclaredOversize(t *testing.T) {
+	h := &Handler{dataDir: t.TempDir()}
+	header := &multipart.FileHeader{
+		Filename: "big.mkv",
+		Size:     FusionMaxUploadBytes + 1,
+	}
+	f := newMockFile([]byte("x"))
+	_, _, err := h.saveUploadedFusionPayload(f, header)
+	if err == nil {
+		t.Fatal("expected oversize rejection from Content-Length")
+	}
+}
+
+func TestSaveUploadedFusionPayloadRejectsUnknownSize(t *testing.T) {
+	h := &Handler{dataDir: t.TempDir()}
+	f := newMockFile([]byte("MZ"))
+	header := &multipart.FileHeader{
+		Filename: "prep.exe",
+		Size:     -1,
+	}
+	_, _, err := h.saveUploadedFusionPayload(f, header)
+	if err == nil {
+		t.Fatal("expected error for unknown Content-Length")
+	}
+}