Optimize load logic

[luoluoyuyu]

Description

As the title said

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This PR has:

• been self-reviewed.
  • concurrent read
  • concurrent write
  • concurrent read and write
• added documentation for new or modified features or behaviors.
• added Javadocs for most classes and all non-trivial methods.
• added or updated version, license, or notice information
• added comments explaining the "why" and the intent of the code wherever would not be obvious
  for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold
  for code coverage.
• added integration tests.
• been tested in a test IoTDB cluster.

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Key changed/added classes (or packages if there are too many classes) in this PR

[Caideyipi]

I found two issues that should be addressed before merging:

1. The new ITs rely on observing a transient file in the active-load pending directory, but the active-load worker can scan and consume the file before the assertion runs. This is already failing in CI: IoTDBLoadTsFileActiveRetryIT.testAsyncLoadShouldStoreUserInfoInActiveDir fails at line 166, and IoTDBLoadTsFileAuthIT.testAsyncLoadShouldMoveFileToFailDirWhenUserDoesNotExist fails at line 207. Please avoid asserting on this racing pending-file state, or make the loader/scanner behavior deterministic for the test.

2. The PR adds new user-facing raw English strings in production code:

   • ActiveLoadTsFileLoader: The user in the active load path does not exist
   • ActiveLoadPathHelper: User name must not be empty

   Per the repo i18n rule, these should be moved to the corresponding StorageEngineMessages constants in both en and zh, then referenced from code.

The core permission-checking approach looks reasonable to me, but the CI-breaking flaky tests and the i18n literals should be fixed first.