[Improve][Elasticsearch] Add LocalDateTime serialization test and simplify serializer

[yzeng1618]

Purpose of this pull request

Fixes #9806.

When using MySQL-CDC as source and Elasticsearch as sink, MySQL TIMESTAMP columns are deserialized into JDK8 LocalDateTime and then serialized in ElasticsearchRowSerializer via Temporal#toString(), producing ISO-8601 strings such as 2023-01-01T12:34:56 or 2023-01-01T12:34:56.123456789.

This behaviour is already compatible with Elasticsearch’s default strict_date_optional_time[_nanos] formats, which expect ISO-8601 style timestamps. Changing the pattern to yyyy-MM-dd HH:mm:ss as suggested in #9806 would not be compatible with ES defaults and would require usersto customize index mappings.

In this PR I keep the existing Temporal#toString() behaviour, add a unit test to lock in the ISO-8601 expectation, and slightly simplify the serialization code by removing unnecessary formatters.

Does this PR introduce any user-facing change?

No.

The connector continues to serialize date/time values using Temporal#toString(), as before. The only changes are:

• A new unit test to verify that LocalDateTime is serialized in ISO‑8601 form with a 'T' separator.
• A small internal refactor in ElasticsearchRowSerializer to reuse the existing Temporal#toString() behaviour instead of introducing custom formatters.

Why not use yyyy-MM-dd HH:mm:ss as suggested in #9806?

The pattern yyyy-MM-dd HH:mm:ss is not accepted by Elasticsearch’s default strict_date_optional_time parser. Using it would require users to adjust the format of their date fields in index mappings, and would break out-of-the-box compatibility.

In contrast, ISO-8601 strings produced by LocalDateTime#toString() (for example 2023-01-01T12:34:56 or with fractional seconds) are directly supported by strict_date_optional_time[_nanos], so no mapping changes are needed.

How was this patch tested?

• Added testSerializeLocalDateTimeFieldFormat in ElasticsearchRowSerializerTest to verify that the serialized LocalDateTime uses ISO‑8601 with a 'T' separator.
• Existing tests continue to pass.

Check list

• [ x ] If any new Jar binary package adding in your PR, please add License Notice according
  New License Guide
• If necessary, please update the documentation to describe the new feature. https://github.com/apache/seatunnel/tree/dev/docs
• If necessary, please update incompatible-changes.md to describe the incompatibility caused by this PR.
• If you are contributing the connector code, please check that the following files are updated:
  1. Update plugin-mapping.properties and add new connector information in it
  2. Update the pom file of seatunnel-dist
  3. Add ci label in label-scope-conf
  4. Add e2e testcase in seatunnel-e2e
  5. Update connector plugin_config

[LiJie20190102]

LGTM

[corgy-w]

Thanks for improving

[Carl-Zhou-CN]

+1