Allow ephemeral ports for loopback redirect URIs

Native OAuth clients such as Claude Code publish loopback redirect URIs
without a port (http://localhost/callback) and pick an ephemeral port at
request time, as described in RFC 8252 section 7.3. The authorize flow
matched the requested redirect URI against the registered set with an
exact string comparison, so http://localhost:3118/callback was rejected
with invalid_redirect_uri even for a trusted, allow-listed client.

Make OAuth2Client.IsRedirectURIAllowed the single source of truth for
redirect matching: it keeps exact matching and adds loopback-aware
matching that ignores the port when scheme, host, path, and query agree.
The redundant document-level check and its duplicate loopback helper in
the CIMD resolver are removed, so both the registered-client and CIMD
paths now rely on one matcher.

Also add a pkg/netx package for the loopback helper.

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

pkg/coredata/oauth2_client.go

@@ -19,6 +19,7 @@ import (
 	"errors"
 	"fmt"
 	"maps"
+	"net/url"
 	"slices"
 	"time"
 
@@ -26,6 +27,7 @@ import (
 	"go.gearno.de/kit/pg"
 	"go.probo.inc/probo/pkg/gid"
 	"go.probo.inc/probo/pkg/iam/policy"
+	"go.probo.inc/probo/pkg/netx"
 	"go.probo.inc/probo/pkg/page"
 	"go.probo.inc/probo/pkg/uri"
 )
@@ -54,7 +56,42 @@ type (
 )
 
 func (c *OAuth2Client) IsRedirectURIAllowed(rawURI string) bool {
-	return slices.Contains(c.RedirectURIs, uri.URI(rawURI))
+	if slices.Contains(c.RedirectURIs, uri.URI(rawURI)) {
+		return true
+	}
+
+	// RFC 8252 §7.3: native apps using a loopback redirect URI choose an
+	// ephemeral port at request time, so the port must be ignored when
+	// matching against the registered loopback redirect URIs.
+	requested, err := url.Parse(rawURI)
+	if err != nil || !netx.IsLoopback(requested.Hostname()) {
+		return false
+	}
+
+	for _, registered := range c.RedirectURIs {
+		candidate, err := url.Parse(registered.String())
+		if err != nil {
+			continue
+		}
+
+		if candidate.Scheme != requested.Scheme {
+			continue
+		}
+
+		if !netx.IsLoopback(candidate.Hostname()) {
+			continue
+		}
+
+		if candidate.Hostname() != requested.Hostname() {
+			continue
+		}
+
+		if candidate.Path == requested.Path && candidate.RawQuery == requested.RawQuery {
+			return true
+		}
+	}
+
+	return false
 }
 
 func (c *OAuth2Client) HasGrantType(grantType OAuth2GrantType) bool {