fix(ai-prompt-decorator): keep empty JSON arrays when rewriting the body

[BetterAndBetterII]

Summary

ai-prompt-decorator re-serialized the request through the immutable body proxy. cycle_aware_deep_copy of that proxy keeps no own keys, so cjson wrote {} and empty JSON arrays such as tools: [] or nested required: [] became objects. Upstream Anthropic then rejects the request with 400 tools: Input should be a valid array.

Copy the underlying table (so array_mt survives), encode with kong.tools.cjson, and write the raw body.

Fixes #14983

Test plan

• Unit: decorate an immutable body whose tools and nested required are empty JSON arrays; encoded output keeps []
• Existing prepend/append message-order unit cases still pass

[CLAassistant]

All committers have signed the CLA.

[BetterAndBetterII]

I have read the CLA Document and I hereby sign the CLA