[refactor](be) Split paused query handling logic (#62485)

Issue Number: None

Related PR: None

Problem Summary: Refactor WorkloadGroupMgr::handle_paused_queries by
splitting cleanup, revoke-resume, per-workload-group processing, and
memory-release paths into focused helpers while preserving the existing
paused query behavior.

None

- Test: Unit Test
- `./run-be-ut.sh --run --filter=WorkloadGroupManagerTest.*
-j${DORIS_PARALLELISM}`
- Behavior changed: Yes
- Does this need documentation: No

### What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

### Release note

None

### Check List (For Author)

- Test <!-- At least one of them must be included. -->
    - [ ] 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 <!-- Add your reason?  -->

- Behavior changed:
    - [ ] No.
    - [ ] Yes. <!-- Explain the behavior change -->

- Does this need documentation?
    - [ ] No.
- [ ] Yes. <!-- Add document PR link here. eg:
apache/doris-website#1214 -->

### Check List (For Reviewer who merge this PR)

- [ ] Confirm the release note
- [ ] Confirm test cases
- [ ] Confirm document
- [ ] Add branch pick label <!-- Add branch pick label that this PR
should merge into -->

Author: mrhhsg