Add username to token validate response

- Include username field in validateResponse struct
- Look up account by UUID and populate username on success
- Add username field to Go client TokenClaims struct
- Fix OpenAPI nullable type syntax (use array form)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

clients/go/client.go

@@ -77,6 +77,7 @@ type PublicKey struct {
 type TokenClaims struct {
 	Valid     bool     `json:"valid"`
 	Sub       string   `json:"sub,omitempty"`
+	Username  string   `json:"username,omitempty"`
 	Roles     []string `json:"roles,omitempty"`
 	ExpiresAt string   `json:"expires_at,omitempty"`
 }