fix: CVE-2026-33322 harden OIDC JWT verification

Close the OIDC JWT algorithm confusion flaw in AssumeRoleWithWebIdentity by restoring a JWKS-only verification path. Stop injecting the client secret into the verifier keyring and restrict accepted signing methods to the asymmetric algorithms already supported by the existing JWKS flow.

Add regression coverage to verify HS256 tokens are rejected, RS256 tokens remain valid, and JWKS refresh and retry logic cannot bypass the method allowlist.

Co-authored-by: Codex <codex@openai.com>
Co-authored-by: Claude Code <claude-code@anthropic.com>

internal/config/identity/openid/openid.go

@@ -18,6 +18,7 @@
 package openid
 
 import (
+	"crypto"
 	"crypto/sha1"
 	"encoding/base64"
 	"errors"
@@ -206,7 +207,7 @@ func LookupConfig(s config.Config, transport http.RoundTripper, closeRespFn func
 		ProviderCfgs:       map[string]*providerCfg{},
 		pubKeys: publicKeys{
 			RWMutex: &sync.RWMutex{},
-			pkMap:   map[string]any{},
+			pkMap:   map[string]crypto.PublicKey{},
 		},
 		roleArnPolicyMap: map[arn.ARN]string{},
 		transport:        openIDClientTransport,