[k8s-keystone-auth] Preserve config after failed reload

[immanuwell]

What this PR does / why we need it:

A malformed live policy or sync ConfigMap logs a parse error, but still replaces working config with empty or partial state
As a result, auth can start denying requests or syncing can silently stop

This keeps the last good config when parsing or validation fails. Valid reloads still work, and failures use the existing retry queue.

Which issue this PR fixes(if applicable):

N/A

Special notes for reviewers:

Repro:

1. Start with a valid policy or sync ConfigMap.
2. Set policies to {, or set syncConfig to malformed YAML.
3. Current code logs the parse failure, then logs that config was updated and replaces the active state.

Tests:

go test -race ./pkg/identity/keystone

go test ./pkg/...

make check

Release note:

NONE

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