[fix](fe) Validate user/role existence in SHOW ROW POLICY command

[heguanhui]

What problem does this PR solve?

Issue Number: close #xxx

Problem Summary: SHOW ROW POLICY FOR and SHOW ROW POLICY FOR ROLE do not validate whether the specified user or role exists. This is inconsistent with CREATE ROW POLICY which does validate, and allows confusing behavior where a non-existent user/role is silently accepted.

Release note

SHOW ROW POLICY now reports an error when the specified user or role does not exist.

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test
  • No need to test

• Behavior changed:

  • Yes. SHOW ROW POLICY now throws an error for non-existent user/role instead of silently returning empty results.

• Does this need documentation?

  • No.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[hello-stephen]

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1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
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