[Fix][Connector-Http] fix Invalid mime type

[litiliu]

Purpose of this pull request

close #9362

Does this PR introduce any user-facing change?

How was this patch tested?

UT added

Check list

• 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 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

[Copilot]

Pull Request Overview

This pull request fixes an invalid MIME type issue in the HTTP connector by adjusting how the request body is handled. Key changes include making the addBody method static, removing redundant header addition that set a content encoding, and updating the test to ensure that no manual content encoding is set.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
seatunnel-connectors-v2/connector-http/connector-http-base/src/test/java/org/apache/seatunnel/connectors/seatunnel/http/client/HttpClientProviderTest.java	Added unit tests to assert headers remain unmodified and content encoding is not set
seatunnel-connectors-v2/connector-http/connector-http-base/src/main/java/org/apache/seatunnel/connectors/seatunnel/http/client/HttpClientProvider.java	Made addBody static and removed redundant and potentially problematic header modifications

Comments suppressed due to low confidence (1)

seatunnel-connectors-v2/connector-http/connector-http-base/src/main/java/org/apache/seatunnel/connectors/seatunnel/http/client/HttpClientProvider.java:492

• Consider adding a comment here explaining that the content encoding is intentionally not set to avoid triggering an invalid MIME type, which will help future maintainers understand the rationale behind this change.

StringEntity entity = new StringEntity(JsonUtils.toJsonString(body), ContentType.APPLICATION_JSON);