[opt](explode) Optimize explode_outer and posexplode (#62069)

[jacktengg]

Issue Number: Pick #62069

Related PR: #xxx

Problem Summary:

Improvement of #60352, optimize explode_outer, posexplode and posexplode_outer.

The approach is a single-pass algorithm: walk through child rows, accumulating contiguous segments into the output, then when hitting a null/empty row or reaching the end, flush the segment using bulk operations. For outer-null rows, insert a NULL and copy the non-table-function columns directly. This naturally handles both outer and
non-outer modes since non-outer mode just won't produce any null outputs. For posexplode, generate position indices alongside this.

Performance test result.
Create and populate a large test table. Use 100,000,000 base rows, each with an array of 10 INT elements, producing 1,000,000,000 total exploded output rows.

Function	slow path	optimized	SpeedUp
explode	9734 ms	5105 ms	90%
explode_outer	9685 ms	5064 ms	91%
posexplode	11021 ms	5963 ms	84%
posexplode_outer	14088 ms	5903 ms	138%

None

• 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.

• Confirm the release note

• Confirm test cases

• Confirm document

• Add branch pick label

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

Release note

None

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?

[jacktengg]

run buildall

[github-actions]

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

[github-actions]

PR approved by anyone and no changes requested.

[hello-stephen]

BE UT Coverage Report

Increment line coverage 88.72% (118/133) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	52.96% (19828/37438)
Line Coverage	36.46% (186446/511326)
Region Coverage	32.72% (144409/441323)
Branch Coverage	33.90% (63394/186990)

[hello-stephen]

BE Regression && UT Coverage Report

Increment line coverage 87.10% (189/217) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	73.18% (26799/36620)
Line Coverage	56.62% (288393/509304)
Region Coverage	54.19% (241148/445019)
Branch Coverage	55.81% (104606/187426)