Support Azure Managed Redis alongside Azure Cache for Redis

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## Summary
Update the SimpleChat Redis implementation so the application fully supports Azure Managed Redis while continuing to support Azure Cache for Redis during the transition period.

## User Value
Admins should be able to configure, view, validate, and operate Redis-backed features regardless of whether the deployment uses Azure Managed Redis or the older Azure Cache for Redis service.

## Proposed Behavior
SimpleChat should recognize and work with Azure Managed Redis across runtime configuration, connectivity, diagnostics, and admin portal surfaces. Existing Azure Cache for Redis deployments should remain supported until a planned retirement point, currently expected around September 2027 unless the timeline changes.

## Acceptance Criteria
- [ ] Runtime Redis connectivity works with Azure Managed Redis.
- [ ] Runtime Redis connectivity continues to work with Azure Cache for Redis during the transition window.
- [ ] Admin portal Redis configuration, status, and viewing surfaces correctly handle both Azure Managed Redis and Azure Cache for Redis.
- [ ] Any provider-specific settings, validation, labels, and help text distinguish the two services clearly.
- [ ] Tests or validation coverage confirm both Redis implementations work.
- [ ] Documentation states the support timeline and expected retirement target for Azure Cache for Redis.

## Notes
Priority: P1
Assignee: Unassigned
Transition target: continue supporting Azure Cache for Redis until at least September 2027, then reassess or remove support as a separate planned change.

Start by tracing the SimpleChat Redis implementation across runtime configuration, connectivity, diagnostics, and admin portal surfaces, then compare how Azure Cache for Redis is currently handled. Review existing tests or validation coverage for Redis providers. Done means both Azure services work, are clearly distinguished in settings and help text, and the documentation records the Azure Cache for Redis support timeline.

Written by the indexing model from the issue text.