test: add missing tests and fix hanging transport test

New test packages:
- config/loader_test.go: 5 tests (valid, minimal, missing file, pulsar-only, defaults)
- serial/serial_test.go: 6 tests (interface, read, EOF, close, empty)
- app/app_test.go: 5 tests (New, dispatch store/send, error handling, ctx cancellation)

Enhanced test packages:
- storage/store_test.go: +3 tests (concurrent insert, close/reopen, init clears)
- telegram/parser_test.go: +5 tests (concurrency, rawLog, empty, multiline, removeEmpty)
- transport/transport_test.go: +7 tests, +fixes

Fixes:
- app.go: store field type *storage.Store -> storage.Repository (supports mocking)
- transport_test: fix net.Pipe() sync deadlock in SetConnClosesPrevious
- transport_test: fix MultiSender_ErrorPropagation expectations

Results: 6 packages, 30+ tests, all pass with -race

app/app.go

@@ -27,7 +27,7 @@ type App struct {
 	rawLog  *rotatelogs.RotateLogs
 	port    *serial.Port
 	parser  *telegram.Parser
-	store   *storage.Store
+	store   storage.Repository
 	sender  transport.Sender
 	tcpSrv  *transport.TCPServer