chore(batch): keep the policy snapshot out of msgp generation

batchPolicySnapshot holds the IAM store, a context and a mutex. Without
an ignore directive, go generate emitted msgp encoders for it that do not
compile, so make check-gen (and the check-gen step of CI) failed on the
generated batch-authz files. The committed generated files are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Feng Ruohang <rh@vonng.com>
(cherry picked from commit a6571a53e9)

cmd/batch-authz.go

@@ -572,6 +572,8 @@ const batchIAMLoadTimeout = 5 * time.Second
 // waiting for the cache lock, which taking the write lock back after each
 // read would cause. It writes nothing to the cache but the lazily loaded
 // mappings, which the request path stores under the read lock as well.
+//
+//msgp:ignore batchPolicySnapshot
 type batchPolicySnapshot struct {
 	*IAMStoreSys
 	ctx   context.Context