Decouple PubSub from Redis in the Broker interface

[halomanlodestar]

A sub part of #173

Summary

This change addresses the TODO on CancelationPubSub() in Broker interface at internal/base/base.go:726.
Removes the hard dependency on *redis.PubSub from the Broker interface, replacing it with a minimal PubSub interface. This is a prerequisite for supporting non-Redis broker implementations.

Motivation

CancelationPubSub() in the Broker interface previously returned *redis.PubSub directly, making it impossible to implement a compliant broker without Redis. This change abstracts that dependency behind an interface.

Changes

New base.PubSub interface

• Defines a broker-agnostic PubSub interface with Channel() <-chan string, and Close
• Broker.CancelationPubSub() now returns (PubSub, error) instead of (*redis.PubSub, error)

New RedisPubSub adapter

• Wraps *redis.PubSub to implement base.PubSub
• Channel() returns <-chan string (message payloads only), hiding Redis-specific message types from callers
• Updated consumers

Testing

Existing TestCancelationPubSub updated to match the new <-chan string API (reads msg directly instead of msg.Payload).

[codecov]

Codecov Report

❌ Patch coverage is 82.35294% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.44%. Comparing base (f8d6677) to head (fbed329).
⚠️ Report is 21 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/rdb/rdb_pubsub.go	84.61%	2 Missing ⚠️
internal/testbroker/testbroker.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1122      +/-   ##
==========================================
+ Coverage   63.34%   63.44%   +0.10%     
==========================================
  Files          29       30       +1     
  Lines        4984     4998      +14     
==========================================
+ Hits         3157     3171      +14     
- Misses       1549     1550       +1     
+ Partials      278      277       -1     

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[kamikazechaser]

This is out of scope. See #1138.