Add OAuth2 API scope registration and enforcement

Register v1 API scopes in coredata, advertise them in OIDC discovery
and protected-resource metadata, show them on the consent screen, and
enforce scope-to-action mapping in the IAM Authorizer before policy
evaluation.

Signed-off-by: Ludovic Vielle <ludovic@probo.com>

pkg/iam/oauth2/pkce_test.go

@@ -0,0 +1,158 @@
+// Copyright (c) 2026 Probo Inc <hello@probo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package oauth2_test
+
+import (
+	"crypto/sha256"
+	"encoding/base64"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.probo.inc/probo/pkg/coredata"
+	"go.probo.inc/probo/pkg/iam/oauth2"
+)
+
+func computeS256Challenge(verifier string) string {
+	h := sha256.Sum256([]byte(verifier))
+	return base64.RawURLEncoding.EncodeToString(h[:])
+}
+
+func TestValidateCodeChallenge(t *testing.T) {
+	t.Parallel()
+
+	verifier := "dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk"
+	challenge := computeS256Challenge(verifier)
+
+	t.Run(
+		"valid s256",
+		func(t *testing.T) {
+			t.Parallel()
+
+			result := oauth2.ValidateCodeChallenge(
+				verifier,
+				challenge,
+				coredata.OAuth2CodeChallengeMethodS256,
+			)
+
+			require.True(t, result)
+		},
+	)
+
+	t.Run(
+		"wrong verifier",
+		func(t *testing.T) {
+			t.Parallel()
+
+			result := oauth2.ValidateCodeChallenge(
+				"wrong-verifier",
+				challenge,
+				coredata.OAuth2CodeChallengeMethodS256,
+			)
+
+			assert.False(t, result)
+		},
+	)
+
+	t.Run(
+		"wrong challenge",
+		func(t *testing.T) {
+			t.Parallel()
+
+			result := oauth2.ValidateCodeChallenge(
+				verifier,
+				"wrong-challenge",
+				coredata.OAuth2CodeChallengeMethodS256,
+			)
+
+			assert.False(t, result)
+		},
+	)
+
+	t.Run(
+		"unsupported method",
+		func(t *testing.T) {
+			t.Parallel()
+
+			result := oauth2.ValidateCodeChallenge(
+				verifier,
+				challenge,
+				coredata.OAuth2CodeChallengeMethod("plain"),
+			)
+
+			assert.False(t, result)
+		},
+	)
+
+	t.Run(
+		"empty method",
+		func(t *testing.T) {
+			t.Parallel()
+
+			result := oauth2.ValidateCodeChallenge(
+				verifier,
+				challenge,
+				coredata.OAuth2CodeChallengeMethod(""),
+			)
+
+			assert.False(t, result)
+		},
+	)
+
+	t.Run(
+		"empty verifier",
+		func(t *testing.T) {
+			t.Parallel()
+
+			result := oauth2.ValidateCodeChallenge(
+				"",
+				challenge,
+				coredata.OAuth2CodeChallengeMethodS256,
+			)
+
+			assert.False(t, result)
+		},
+	)
+
+	t.Run(
+		"empty challenge",
+		func(t *testing.T) {
+			t.Parallel()
+
+			result := oauth2.ValidateCodeChallenge(
+				verifier,
+				"",
+				coredata.OAuth2CodeChallengeMethodS256,
+			)
+
+			assert.False(t, result)
+		},
+	)
+
+	t.Run(
+		"both empty",
+		func(t *testing.T) {
+			t.Parallel()
+
+			result := oauth2.ValidateCodeChallenge(
+				"",
+				"",
+				coredata.OAuth2CodeChallengeMethodS256,
+			)
+
+			assert.False(t, result)
+		},
+	)
+}