[fix](fe) Prune empty and cascading CTE plans

[englefly]

What problem does this PR solve?

Issue Number: None

Related PR: None

Problem Summary: Fix optimizer-side CTE pruning so empty-relation producers, zero-consumer anchors, and cascading inline opportunities are normalized to a fixpoint before memoization, and add regression coverage for empty and cascading CTE elimination.

Release note

None

Check List (For Author)

• Test: No need to test (user requested to skip compile and test)
• Behavior changed: Yes (empty and cascade-pruned CTEs are normalized before memoization)
• 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.
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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?

[englefly]

/review

[englefly]

run buildall

[hello-stephen]

FE UT Coverage Report

Increment line coverage 82.00% (41/50) 🎉
Increment coverage report
Complete coverage report

[github-actions]

OpenCode automated review failed and did not complete.

Error: Review step was failure (possibly timeout or cancelled)
Workflow run: https://github.com/apache/doris/actions/runs/24895847727

Please inspect the workflow logs and rerun the review after the underlying issue is resolved.

[hello-stephen]

FE Regression Coverage Report

Increment line coverage 90.00% (45/50) 🎉
Increment coverage report
Complete coverage report