cleanup(pubsub): tighten visibility of internal dispatcher components

[suzmue]

Tighten visibility from pub(crate) to private for internal actor components and helper functions that are exclusively used within publisher/actor.rs and subscriber/handler.rs.

[gemini-code-assist]

Code Review

This pull request restricts the visibility of several internal components within the pubsub crate by changing pub(crate) modifiers to private. These changes affect the publisher's actor implementations and the subscriber's acknowledgement handling logic. I have no feedback to provide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.76%. Comparing base (ac2237e) to head (15a9498).
⚠️ Report is 20 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5633      +/-   ##
==========================================
- Coverage   97.76%   97.76%   -0.01%     
==========================================
  Files         220      220              
  Lines       50961    50957       -4     
==========================================
- Hits        49823    49819       -4     
  Misses       1138     1138              

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