[draft](connectionContext) Fix potential memory leak by ensuring ConnectContext#cleanup() is called when ConnectionContext is no longer in use

[seawinde]

What problem does this PR solve?

ConnectionContext maintains an aliveSessionSet in the Env class. The session ID associated with a ConnectionContext is only removed from this set when ConnectContext#cleanup() is explicitly called.

In certain scenarios—such as when a ConnectionContext is no longer in use—if only ConnectContext#remove() is invoked without calling ConnectContext#cleanup(), the session ID remains in Env#aliveSessionSet, causing it to grow indefinitely and eventually leading to a memory leak.

Therefore, whenever a ConnectionContext is confirmed to be no longer needed, ConnectContext#cleanup() must be called (e.g., in AutoCloseConnectContext#close()). On the other hand, if the ConnectionContext will still be used afterward, it's appropriate to call only ConnectContext#remove() (e.g., in AutoCloseSessionVariable).

This PR standardizes the usage of these methods across the codebase to prevent potential memory leaks.

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

Release note

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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.
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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?

[seawinde]

run buildall

[hello-stephen]

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