fix: use plain strings for Keycloak client attributes in update payload

[akram]

Summary

• mergeDesiredClientIntoMap was wrapping attribute values in []interface{} arrays (e.g. ["false"] instead of "false")
• Keycloak Admin API rejects this with 400: Cannot parse the JSON on PUT
• This caused operator-managed client registration to silently fail for all workloads when the Keycloak client already existed and needed an update
• Credential secrets were never created, leaving pods stuck in Init:0/1 waiting for volume mounts

Root Cause

In internal/keycloak/admin.go, mergeDesiredClientIntoMap line 414:

// Before (broken):
attrs[k] = []interface{}{v}

// After (fixed):
attrs[k] = v

How it was found

Discovered while testing PR #321 on an OpenShift cluster. The operator authenticated to Keycloak successfully but every client update returned 400 "Cannot parse the JSON". Reproduced by manually PUTting the same payload to the Keycloak Admin API — array-wrapped attributes fail, plain string attributes succeed.

Test plan

• go test ./internal/keycloak/... — all 11 tests pass (updated TestAdmin_RegisterOrFetchClient_updatesDrift)
• Verified on live OpenShift cluster: client registration succeeds, credential secrets created, pods start

[cwiklik]

Clean one-line fix for a real bug — Keycloak rejects array-wrapped attributes with 400. Root cause well-documented, test updated, all CI green.

Areas reviewed: Go
Commits: 1, signed-off
CI: all passing

[pdettori]

/lgtm