[fix](fe) Serialize workload policy session updates

[feiniaofeiafei]

What problem does this PR solve?

Issue Number: N/A

Related PR: N/A

Problem Summary: Workload policies updated a connection SessionVariable from the policy daemon while the connection thread could record or revert statement-scoped variables. Concurrent access to sessionOriginValue could throw ConcurrentModificationException, and a policy value could be overwritten by statement cleanup. Serialize session variable access for each command, apply policy updates only while the connection is idle, and explicitly exclude policy updates from statement rollback tracking.

Release note

None

Check List (For Author)

• Test: Unit Test
  • ./run-fe-ut.sh --run org.apache.doris.qe.ConnectContextTest
  • mvn checkstyle:check -pl fe-core
• Behavior changed: Yes (policy session-variable updates wait for an idle connection)
• 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

  • 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.
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?