fix: isolate logger response body buffers

[nic-6443]

This fixes response body collection when loggers are configured in multiple scopes for the same request. Each logger configuration now keeps an isolated body buffer, so a route-level logger cannot overwrite the response body collected for a global logger.

The change keeps existing plugin configuration unchanged and preserves the current response body size limit behavior.

Tests:

• prove -I. -Itest-nginx/lib -r t/core/response.t t/plugin/http-logger.t

[Copilot]

Pull request overview

Fixes a bug where multiple logger plugins (e.g., a global http-logger and a route-level file-logger) collecting the response body would share ctx._resp_body_bytes and ctx._body_buffer[ctx._plugin_name], allowing one logger's accumulation/truncation state to break another logger's collection. The fix isolates the body buffer per caller by accepting an optional body_buffer_key (the logger's conf table) and moves byte counting onto the buffer itself, while also introducing a done flag so truncated buffers stop accumulating without dropping subsequent loggers' data.

Changes:

• core.response.hold_body_chunk now accepts a body_buffer_key and stores per-key bytes/done instead of a request-wide ctx._resp_body_bytes.
• log-util.collect_body passes the plugin conf as the buffer key and removes the now-redundant pre-call byte check.
• New t/plugin/http-logger.t tests 28–29 cover global http-logger + global file-logger + route-level file-logger all collecting bodies simultaneously.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
apisix/core/response.lua	Adds body_buffer_key param, per-buffer byte/done tracking.
apisix/utils/log-util.lua	Uses plugin conf as the buffer key and drops the shared-counter guard.
t/plugin/http-logger.t	Adds regression test for global+route loggers collecting response body.

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