[refactor](be) Replace std::unique_ptr with DorisUniqueBufferPtr in SingleValueDataString (#62374)

[mrhhsg]

Pick #62374

Problem Summary: SingleValueDataString uses std::unique_ptr<char[]> with new[] for large string storage, which bypasses the Doris memory tracking allocator. This replaces it with DorisUniqueBufferPtr so that allocations go through Allocator and are properly tracked.

None

• Test: Unit Test (added SingleValueDataStringTest)
• Behavior changed: No
• Does this need documentation: No

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

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

[mrhhsg]

run buildall

[Thearas]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

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?

[hello-stephen]

BE UT Coverage Report

Increment line coverage 100.00% (3/3) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	52.97% (19878/37524)
Line Coverage	36.54% (187254/512519)
Region Coverage	32.86% (145424/442530)
Branch Coverage	33.99% (63705/187431)

[hello-stephen]

BE Regression && UT Coverage Report

Increment line coverage 100.00% (3/3) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	71.48% (26255/36731)
Line Coverage	54.37% (277737/510802)
Region Coverage	51.69% (230795/446474)
Branch Coverage	53.09% (99786/187957)