Add the provider contract, error taxonomy, naming, and config (Step 2)

The Provider interface (Create/Get/Delete/ListByTag), Instance, and
CreateRequest that every cloud backend implements — kept independent of
api/v1alpha1 so this package has no CRD-type coupling.

Error taxonomy (ErrNotFound/ErrQuotaExceeded/ErrTransient/ErrPermanent)
wrapped via a multi-error Unwrap() []error, so errors.Is and errors.As
both work off the same value: the reconciler branches on classification,
logs keep the underlying SDK error. Unclassified errors default to
ErrTransient — retrying is always safer than latching Failed.

Deterministic instance naming (SHA-256 -> base32 -> 16 chars, 22 total
with the "proxy-" prefix) satisfying GCP's RFC1035 name rules with
headroom, and idempotency-tested across 10k UIDs with zero collisions.

--providers-config YAML parsing (config.go) with fail-fast validation:
unknown type, duplicate name, missing gcp.project, mismatched
type/config-block, and strict-mode rejection of unknown keys.

internal/provider/registry/registry.go takes its type->constructor map
as a parameter rather than hardcoding it, so the package has zero import
on internal/provider/mock or internal/provider/gcp (neither exists yet —
mock is Step 3, gcp is Step 8) and compiles today. Explicit wiring moves
to the composition root in cmd/main.go (Step 10).

Deferred internal/provider/metrics.go (the WithMetrics decorator) to
Step 9, where the Prometheus vectors it needs actually get built —
nothing in this step depends on it.

internal/provider at 96.2% coverage, internal/provider/registry at 100%.
make test green.

Co-Authored-By: Claude <noreply@anthropic.com>

internal/provider/registry/registry_test.go

@@ -0,0 +1,92 @@
+package registry
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"gitea.home.hrajfrisbee.cz/kacerr/egress-proxies-operator/internal/provider"
+)
+
+// stubProvider is a minimal provider.Provider satisfying the interface for
+// registry tests, which care about wiring, not provider behavior.
+type stubProvider struct{ name string }
+
+func (s *stubProvider) Create(ctx context.Context, req provider.CreateRequest) (string, error) {
+	return "", nil
+}
+func (s *stubProvider) Get(ctx context.Context, providerID string) (*provider.Instance, error) {
+	return nil, provider.ErrNotFound
+}
+func (s *stubProvider) Delete(ctx context.Context, providerID string) error { return nil }
+func (s *stubProvider) ListByTag(ctx context.Context) ([]provider.Instance, error) {
+	return nil, nil
+}
+
+func stubConstructor(name string) Constructor {
+	return func(ctx context.Context, cfg provider.ProviderConfig) (provider.Provider, error) {
+		return &stubProvider{name: name}, nil
+	}
+}
+
+func TestBuild_dispatchesByType(t *testing.T) {
+	t.Parallel()
+	cfg := &provider.Config{Providers: []provider.ProviderConfig{
+		{Name: "mock", Type: "mock"},
+		{Name: "gcp-eu", Type: "gcp"},
+	}}
+	builtins := map[string]Constructor{
+		"mock": stubConstructor("mock"),
+		"gcp":  stubConstructor("gcp"),
+	}
+
+	providers, err := Build(context.Background(), cfg, builtins)
+	if err != nil {
+		t.Fatalf("Build() error = %v, want nil", err)
+	}
+	if len(providers) != 2 {
+		t.Fatalf("len(providers) = %d, want 2", len(providers))
+	}
+	if _, ok := providers["mock"]; !ok {
+		t.Error(`providers["mock"] missing`)
+	}
+	if _, ok := providers["gcp-eu"]; !ok {
+		t.Error(`providers["gcp-eu"] missing, keyed by config name not type`)
+	}
+}
+
+func TestBuild_unknownType(t *testing.T) {
+	t.Parallel()
+	cfg := &provider.Config{Providers: []provider.ProviderConfig{
+		{Name: "p1", Type: "azure"},
+	}}
+	_, err := Build(context.Background(), cfg, map[string]Constructor{"mock": stubConstructor("mock")})
+	if err == nil {
+		t.Fatal("Build() error = nil, want error for unknown type")
+	}
+}
+
+func TestBuild_constructorErrorAbortsWholeBuild(t *testing.T) {
+	t.Parallel()
+	failing := errors.New("boom")
+	cfg := &provider.Config{Providers: []provider.ProviderConfig{
+		{Name: "mock", Type: "mock"},
+		{Name: "bad", Type: "bad"},
+	}}
+	builtins := map[string]Constructor{
+		"mock": stubConstructor("mock"),
+		"bad": func(ctx context.Context, cfg provider.ProviderConfig) (provider.Provider, error) {
+			return nil, failing
+		},
+	}
+	providers, err := Build(context.Background(), cfg, builtins)
+	if err == nil {
+		t.Fatal("Build() error = nil, want error from failing constructor")
+	}
+	if !errors.Is(err, failing) {
+		t.Errorf("Build() error = %v, want it to wrap %v", err, failing)
+	}
+	if providers != nil {
+		t.Errorf("Build() providers = %v, want nil on failure (no partial result)", providers)
+	}
+}