Fix PubSub client health check handling for sync client implementation

[petyaslavova]

Description of change

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Fixes #3791

[Copilot]

Pull request overview

This PR fixes a bug in the synchronous PubSub client's health check response handling when decode_responses=True. Previously, the is_health_check_response() method would fail to recognize the string format "redis-py-health-check" that Redis returns when there are no active subscriptions. The fix adds logic to check for the appropriate response format based on the decode_responses setting.

Key changes:

• Modified is_health_check_response() method to handle both list and string formats for both decode_responses=True and decode_responses=False scenarios
• Added comprehensive test coverage for sync PubSub health check response validation
• Added comprehensive test coverage for async PubSub health check response validation

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
redis/client.py	Updated is_health_check_response() method to properly handle health check responses based on decode_responses setting, adding check for string format when decode_responses=True
tests/test_pubsub.py	Added TestPubSubHealthCheckResponse test class with 8 test methods covering all scenarios for health check response validation in sync client
tests/test_asyncio/test_pubsub.py	Added TestPubSubHealthCheckResponse test class with 6 test methods covering all scenarios for health check response validation in async client

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