Fix parquet row group reader lifetime issues

[Gabriel39]

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

[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?

[Gabriel39]

/review

[github-actions]

Review result: no blocking issues found.\n\nCritical checkpoints:\n- Repository instructions: no AGENTS.md or in-repo code-review skill file was present in the workspace; I followed the supplied automated review instructions and existing-review context.\n- Existing review threads: none were present, so no duplicate concerns were submitted.\n- Scope reviewed: full PR diff reviewed; changes are limited to be/src/format/parquet/vparquet_group_reader.cpp and be/src/format/parquet/vparquet_group_reader.h.\n- Correctness/lifetime: moving _column_readers after _read_ranges makes member destruction release readers before _read_ranges, matching the reader reference lifetime requirement. Removing the explicit _column_readers.clear() avoids fighting declaration-order destruction and leaves file_reader/state/raw references with adequate lifetime for reader teardown.\n- Tests/verification: git diff --check passed; no focused runtime test was available from the PR.\n- User focus: no additional user-provided review focus was specified.