fix(request): make add operations append existing values

[SouthwestAsiaFloat]

Description

This PR fixes the inconsistency between add* and set* operations in the Request Plugin.

Previously, addParameters, addHeaders, and addCookies internally used set() operations, causing existing values to be overwritten and making them behave the same as their corresponding set* operations.

This behavior is inconsistent with the documented semantics in RequestHandle and the implementation of ModifyResponsePlugin, where:

• add* operations append values.
• set* operations overwrite values.

Changes

• Use queryParams.add(...) for addParameters.
• Use headers.add(...) for addHeaders.
• Use cookies.add(...) for addCookies.
• Keep existing setParameters, setHeaders, and setCookies behavior unchanged.
• Add/update unit tests covering requests that already contain the target key.

Tests

• ./mvn -pl shenyu-plugin/shenyu-plugin-request test

Checklist

• I have read the contribution guidelines.
• I submit test cases that back my changes.
• My local tests passed.

Fixes #6360