[fix](fe) Add user and role existence check in DropRowPolicyCommand

[heguanhui]

What problem does this PR solve?

Issue Number: close #63322

Problem Summary: DropRowPolicyCommand does not validate whether the user or role specified in the DROP ROW POLICY statement actually exists. This is inconsistent with CreatePolicyCommand, which checks user existence via doesUserExist() and role existence via doesRoleExist(). Without this validation, a typo in the user or role name could silently match no policy, or the user might mistakenly believe the drop succeeded.

Release note

DROP ROW POLICY now validates that the specified user and role exist, returning an error if they do not. This makes the behavior consistent with CREATE ROW POLICY.

Check List (For Author)

• Test
  • Unit Test
• Behavior changed:
  • Yes. DROP ROW POLICY now throws AnalysisException when the specified user or role does not exist, whereas previously it would proceed without validation.
• 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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