[Chore](exchange) reduce create wait_channel_timer

[BiteTheDDDDt]

What problem does this PR solve?

Currently, the number of our wait_channel_timers corresponds to the number of channels. After the modifications in this PR, the number of wait_channel_timers will align with the number of local channels, thereby reducing unnecessary timer generation.

This pull request refactors and improves code readability and safety in the exchange_sink_operator.cpp file by modernizing loop constructs, improving memory management, and enhancing error handling. The changes mainly focus on using range-based loops, smart pointers, and clearer logic for managing channel dependencies and timers.

Code modernization and safety improvements:

• Replaced traditional for-loops with range-based for-loops for iterating over channels, improving readability and reducing indexing errors in multiple methods (init, open). [1] [2]
• Updated memory management for _writer by switching from manual reset with new to std::make_unique, ensuring exception safety and clarity.
• Used contains method instead of find for map lookups in _create_channels, making the code more concise and modern.
• Changed range-based loop in the constructor to use const auto& for safety and clarity.

Dependency and error handling improvements:

• Added a null pointer check for local channel dependencies and improved timer management, ensuring that dependencies are valid and timers are only added for non-null dependencies.

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[Thearas]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[BiteTheDDDDt]

run buildall

[hello-stephen]

BE UT Coverage Report

Increment line coverage 78.57% (11/14) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	52.73% (18079/34287)
Line Coverage	38.05% (164452/432241)
Region Coverage	32.36% (125314/387250)
Branch Coverage	33.80% (54892/162387)

[hello-stephen]

BE Regression && UT Coverage Report

Increment line coverage 85.71% (12/14) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	71.49% (24095/33706)
Line Coverage	57.91% (250730/432986)
Region Coverage	52.82% (207496/392846)
Branch Coverage	54.67% (89388/163496)

[github-actions]

PR approved by anyone and no changes requested.

[github-actions]

PR approved by at least one committer and no changes requested.