Add storage tests for URL parsing and every in-process backend (closes #66)

internal/storage had no tests. This adds table-driven ParseStorageURL
coverage and a shared Storer conformance suite, defined once in
conformance_test.go and run against every backend that can run
in-process: file:// over a temp dir and s3:// over the same in-memory
S3 harness (gofakes3 + s3mem) that internal/s3 and the not-found test
use, so a new backend inherits the contract by passing its constructor.
The rclone backend gets construction and argument-shaping tests via
rclone's in-process ":local" backend, plus the missing-remote error
path; a comment records that its data-plane operations need a configured
remote and so cannot run in a unit test.

Tests only; no production code changed and no defect surfaced.

Model: opus-4-8

internal/storage/rclone_test.go

@@ -0,0 +1,58 @@
+package storage_test
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"sneak.berlin/go/vaultik/internal/storage"
+)
+
+// The rclone backend is a thin adapter over the rclone library: it turns a
+// (remote, path) pair into rclone's "remote:path" string, hands it to
+// rclone, and maps rclone's own results back to the Storer interface. What
+// can be tested in-process, without a configured remote or network, is that
+// adapter layer — how the arguments are shaped and how construction errors
+// are reported. The data-plane operations (Put/Get/List/Delete) are rclone's
+// own, exercised against a real provider (drive, s3-via-rclone, ...), which
+// needs a configured remote with credentials and network access and so is
+// out of reach of a unit test. The shared Storer conformance suite therefore
+// runs against the in-process file and s3 backends; the rclone backend
+// inherits that contract once a remote is configured.
+//
+// These tests use rclone's ":local:" on-the-fly backend, which addresses the
+// local filesystem directly without any configured remote, so construction
+// runs entirely in-process.
+
+// TestNewRcloneStorerConstruction checks that a valid remote constructs a
+// backend and that Info() reports the shaped "remote:path" location.
+//
+//nolint:paralleltest // NewRcloneStorer installs the process-global rclone config
+func TestNewRcloneStorerConstruction(t *testing.T) {
+	dir := t.TempDir()
+
+	s, err := storage.NewRcloneStorer(context.Background(), ":local", dir)
+	if err != nil {
+		t.Fatalf("NewRcloneStorer: %v", err)
+	}
+
+	// Info().Location is the "remote:path" string the adapter builds from
+	// its two arguments, so asserting it confirms the argument shaping.
+	want := ":local:" + dir
+	if got := s.Info().Location; got != want {
+		t.Errorf("Info().Location = %q, want %q", got, want)
+	}
+}
+
+// TestNewRcloneStorerUnknownRemote checks that a remote that is not in the
+// rclone config fails construction with the ErrRemoteNotFound sentinel,
+// rather than silently returning a backend pointed nowhere.
+//
+//nolint:paralleltest // NewRcloneStorer installs the process-global rclone config
+func TestNewRcloneStorerUnknownRemote(t *testing.T) {
+	_, err := storage.NewRcloneStorer(
+		context.Background(), "vaultik-no-such-remote", "path")
+	if !errors.Is(err, storage.ErrRemoteNotFound) {
+		t.Errorf("NewRcloneStorer error = %v, want ErrRemoteNotFound", err)
+	}
+}