refactor/fix: stream subscriptions in RedisConnector

[d-perl]

Refactors the Redis stream subscription logic in the RedisConnector

• unifies all "normal" stream subscriptions: those which should read from the start of the stream are migrated to the normal subscriptions after the first read attempt, by reading up to the place those streams have been read already in the normal subscriptions, if it exists
• clarifies the organisation by moving the subscription logic methods to a separate class
• fixes some logical mistakes:
  • it is an error to register the same stream with the same callback more than once
  • no longer blocks separately on from_start streams after the first try
  • some small errors in the tests

Looks like some small patch in BEC widgets will be required where duplicate subs were previously ignored - workflow against BW branch https://github.com/bec-project/bec/actions/runs/23142709927

closes #790

[codecov]

Codecov Report

❌ Patch coverage is 83.24607% with 32 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
bec_lib/bec_lib/redis_connector.py	83.24%	19 Missing and 13 partials ⚠️

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