fix(ai-prompt): preserve all original messages in decorateBody

[eye-gu]

close #6342

AiPromptPlugin#decorateBody used messages.get(0) which only kept the first message, silently dropping all remaining conversation history (multi-turn dialogue messages).

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• You submit test cases (unit or integration tests) that back your changes.
• Your local test passed ./mvnw clean install -Dmaven.javadoc.skip=true.

[Copilot]

Pull request overview

Fixes AiPromptPlugin#decorateBody so it preserves the full incoming messages array (multi-turn conversation history) instead of only keeping the first entry, aligning behavior with expected AI chat request semantics (issue #6342).

Changes:

• Update decorateBody to append all original messages via decoratedMessages.addAll(messages).
• Add unit tests covering decoration behavior (no messages, empty messages, prepend/append, multi-message preservation, and preservation of other JSON fields).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
shenyu-plugin/shenyu-plugin-ai/shenyu-plugin-ai-prompt/src/main/java/org/apache/shenyu/plugin/ai/prompt/AiPromptPlugin.java	Fixes message decoration to preserve all original conversation messages.
shenyu-plugin/shenyu-plugin-ai/shenyu-plugin-ai-prompt/src/test/java/org/apache/shenyu/plugin/ai/prompt/AiPromptPluginTest.java	Adds coverage to validate correct body decoration and multi-message preservation.

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