[IOTDB-17433] Fix timezone offset bug in DateTimeUtils

[Lexert19]

Description

Refactored DateTimeUtils.toZoneOffset to calculate the timezone offset based on the provided string instead of the current system time (Instant.now()).
Added comprehensive unit tests in DateTimeUtilsTest to cover DST transitions.
Fixes #17433

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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