Add Sentinel mode unit tests for Valkey client reconnection

[jopemachine]

Objective

Add unit tests verifying Valkey client behavior in Sentinel mode, covering master failover detection and reconnection scenarios that are not testable with a standalone Redis instance.

Background

Currently, all Valkey client tests use a standalone Redis container. There are no tests for Sentinel-specific behavior such as master address change detection (need_reconnect), Sentinel discovery failure handling, or reconnection after master failover. BA-5577 requires verification in both Standalone and Sentinel modes.

Scope

Mock-based unit tests

• ValkeySentinelClient.need_reconnect() detects master address change
• ValkeySentinelClient._get_master_address() handles Sentinel discovery failure
• MonitoringValkeyClient monitor loop triggers reconnect on master change (via mocked Sentinel)
• acquire() failure tracking and reconnect with Sentinel client (via mocked Sentinel)

Mock targets

• Sentinel.discover_master() — control master address returns
• GlideClient.create() — return mock client without real connection

Future consideration

Integration tests with real Sentinel infrastructure (Docker Compose with master + replica + sentinel) would require CI infrastructure changes and are out of scope for this issue.

Acceptance Criteria

• Unit tests for ValkeySentinelClient Sentinel-specific behavior (need_reconnect, master discovery failure)
• Unit tests for MonitoringValkeyClient wrapping a Sentinel client (monitor loop + acquire reconnect)
• All tests pass in CI

JIRA Issue: BA-5585