Extend OAuth2 CIMD for compliance portal clients

Teach CIMD registration and discovery about per-portal client
metadata, and carry portal context through token and ID token
issuance for downstream session creation.

Signed-off-by: Bryan Frimin <bryan@probo.com>

pkg/iam/oauth2/cimd_compliance_portal_test.go

@@ -0,0 +1,35 @@
+// 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
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCIMDClientIDHost(t *testing.T) {
+	t.Parallel()
+
+	host, ok := CIMDClientIDHost("https://portal.example.com/.well-known/oauth-client-metadata")
+	assert.True(t, ok)
+	assert.Equal(t, "portal.example.com", host)
+
+	_, ok = CIMDClientIDHost("https://chatgpt.com/oauth/client.json")
+	assert.True(t, ok)
+
+	_, ok = CIMDClientIDHost("not-a-url")
+	assert.False(t, ok)
+}