[ISSUE #4105]♻️Clean up existing clippy warnings of new_without_default, field_reassign_with_default, const_is_empty, large_enum_variant

[WaterWhisperer]

Which Issue(s) This PR Fixes(Closes)

#4087 Not complete yet
fix #4105

Brief Description

Clean up existing clippy warnings of types:

• new_without_default
• field_reassign_with_default
• const_is_empty
• large_enum_variant

How Did You Test This Change?

[rocketmq-rust-bot]

🔊@WaterWhisperer 🚀Thanks for your contribution🎉！

💡CodeRabbit(AI) will review your code first🔥！

Note

🚨The code review suggestions from CodeRabbit are to be used as a reference only, and the PR submitter can decide whether to make changes based on their own judgment. Ultimately, the project management personnel will conduct the final code review💥.

[Copilot]

Pull Request Overview

This PR addresses Rust Clippy warnings to improve code quality by fixing specific warning types: new_without_default, field_reassign_with_default, const_is_empty, and large_enum_variant.

• Wraps large enum variant in Box to reduce memory usage
• Replaces mutable field reassignment patterns with struct initialization syntax
• Adds Default implementations for structs with new() methods
• Removes redundant constant emptiness tests

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
rocketmq-store/src/message_store.rs	Boxes large enum variant to reduce size
rocketmq-store/src/ha/flow_monitor.rs	Replaces field reassignment with struct initialization
rocketmq-namesrv/src/processor/default_request_processor.rs	Replaces field reassignment with struct initialization in tests
rocketmq-client/examples/ordermessage/ordermessage_consumer.rs	Adds Default implementation for MyMessageListener
rocketmq-broker/src/metrics/pop_metrics_constant.rs	Removes redundant const emptiness tests
rocketmq-broker/src/metrics/broker_metrics_constant.rs	Removes redundant const emptiness tests
rocketmq-broker/benches/syncunsafecell_mut.rs	Adds Default implementation for Test struct

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 26.55%. Comparing base (c2ae50a) to head (bfdb3bb).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4100      +/-   ##
==========================================
- Coverage   26.56%   26.55%   -0.01%     
==========================================
  Files         575      575              
  Lines       81359    81356       -3     
==========================================
- Hits        21609    21606       -3     
  Misses      59750    59750              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.