[Chore](exchange) call add_rows with data and eos at once

[BiteTheDDDDt]

What problem does this PR solve?

This pull request simplifies the logic for handling end-of-stream (EOS) conditions in the _channel_add_rows method of the Status Writer class. The change ensures that EOS is processed together with regular row additions, removing redundant code and improving maintainability.

Refactoring EOS handling:

• The EOS logic is now integrated into the main loop, so channels receive EOS flags along with regular row additions, eliminating the need for a separate loop for EOS. (be/src/pipeline/shuffle/writer.cpp, be/src/pipeline/shuffle/writer.cppL99-L112)

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

[hello-stephen]

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 Regression && UT Coverage Report

Increment line coverage 100.00% (2/2) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	71.45% (24421/34180)
Line Coverage	57.98% (254444/438849)
Region Coverage	53.17% (212297/399283)
Branch Coverage	54.60% (90604/165932)