Replaces per-invocation new ObjectMapper() in the accessDeniedHandler
lambda with a static field (avoids repeated allocation). ObjectMapper
cannot be injected in SecurityConfig because @WebMvcTest slices exclude
JacksonAutoConfiguration; the static instance is safe since the response
only serialises fixed String keys.
Also corrects the ADR cross-reference in the CSRF comment from ADR-020
(Spring Session JDBC) to ADR-022 (CSRF + session revocation).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>