branch-4.1: [fix](fe) Reject invalid stream load tokens on commit and rollback

[liaoxin01]

Pick #62111

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[Copilot]

Pull request overview

Backport of #62111 to FE that enforces stream-load/txn commit & rollback token validation, preventing invalid tokens from being accepted in these finalization RPC paths.

Changes:

• Switch token-handling in load/txn commit and rollback implementations to a throwing validation (checkTokenOrThrow) instead of ignoring checkToken’s boolean result.
• Add checkTokenOrThrow(String) helper that raises AuthenticationException on invalid tokens.
• Add unit tests covering invalid-token rejection for loadTxnCommit/loadTxnRollback/commitTxn/rollbackTxn, and ensuring the error message doesn’t echo the raw token.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
fe/fe-core/src/main/java/org/apache/doris/service/FrontendServiceImpl.java	Enforces token validation on commit/rollback paths via a new throwing helper.
fe/fe-core/src/test/java/org/apache/doris/service/FrontendServiceImplTest.java	Adds regression tests ensuring invalid tokens are rejected on commit/rollback operations.

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[liaoxin01]

run buildall