fix: sonic ast cache need clean after middleware

[zijiren233]

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[Copilot]

Pull request overview

This PR addresses memory/cleanup concerns around Bytedance Sonic AST usage by switching to explicit GetWithOptions(..., ast.SearchOptions{}) parsing and by ensuring cached request-body AST nodes and request-body readers are properly released/cleared during relay handling.

Changes:

• Replace multiple sonic.Get(...) calls with sonic.GetWithOptions(..., ast.SearchOptions{}) across plugins/adaptors to avoid unwanted AST caching behavior.
• Ensure converted outbound request bodies are closed in prepareAndDoRequest, including early-error paths; add regression tests.
• Change request-body caching to use a reusable request body wrapper and explicitly clear the per-request cached AST node key in middleware.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
core/relay/plugin/web-search/search.go	Use GetWithOptions when parsing response JSON for metadata injection.
core/relay/plugin/thinksplit/split.go	Use GetWithOptions for safer streaming chunk parsing; add ast import.
core/relay/plugin/streamfake/fake.go	Use GetWithOptions when parsing streaming chunks into AST nodes.
core/relay/plugin/cachefollow/cachefollow.go	Use GetWithOptions for parsing retention fields from SSE/JSON.
core/relay/controller/dohelper.go	Close converted request bodies reliably; refine body-detail truncation helpers.
core/relay/controller/dohelper_test.go	Add tests asserting converted request bodies are closed on success/failure paths.
core/relay/adaptor/siliconflow/adaptor.go	Use IsSuccessfulResponseStatus for mode-aware success codes.
core/relay/adaptor/openai/stt.go	Use GetWithOptions when injecting usage into SSE JSON payloads.
core/relay/adaptor/openai/image.go	Use GetWithOptions for parsing image stream SSE JSON payloads.
core/relay/adaptor/openai/chat.go	Use GetWithOptions for parsing chat stream SSE JSON payloads.
core/relay/adaptor/anthropic/main.go	Use GetWithOptions for parsing Claude stream SSE payloads.
core/middleware/distributor.go	Clear cached request-body AST node before handing off to downstream handlers.
core/middleware/ctxkey.go	Make request-body-node cache key internal (requestBodyNode).
core/common/body.go	Rework request body caching via reusable body wrapper; use GetWithOptions for node extraction.
core/common/body_test.go	Update expectations to match new request body caching behavior (no GetBody).

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