fix: isolate callback failures from HTTP workers

[hjianbo]

Summary

This is a robustness fix for the ehttpc worker pool.

Async callbacks are provided by callers, so an exception in callback code must not be able to take down the HTTP transport worker. Before this change, callbacks were evaluated directly in the ehttpc worker process. An unexpected upstream response could therefore kill a worker, and repeated failures could exhaust the supervisor restart intensity and leave the whole pool unavailable until the resource was restarted.

The fix keeps callback failures contained while preserving the existing request representations on both synchronous and asynchronous hot paths. Invalid callback shapes are rejected before dispatch, and malformed callback values cannot be passed to gen_server:reply/2. Callback failures are logged without including the HTTP response payload.

What changed

• Validate the callback shape before sending an async request.
• Distinguish callbacks from synchronous replies with guarded dispatch, without adding target wrappers to request state.
• Catch callback failures from response, transport error, timeout, and stream-ref paths.
• Preserve the existing callback forms, argument ordering, and execution timing for valid callers.
• Add regression coverage for callback failures, malformed callbacks, timeout callbacks, multi-argument callbacks, and repeated failures.

Reference incident

The business-level response parsing problem is tracked separately in emqx/emqx_whc#142. This PR is the ehttpc-side robustness fix.

The following redacted excerpts from that incident show why callback failures must not be allowed to terminate the shared pool:

2026-08-29T10:39:07.537453+08:00 [error]
Supervisor: {<...>,ehttpc_worker_sup}.
Context: child_terminated.
Reason: {{case_clause,{ok,#{<<"code">> => 0,<<"data">> => [...]}}}, ...}.
Offender: id={whc_http_dm_pool,...}

2026-08-29T10:39:12.230851+08:00 [error]
Supervisor: {<...>,ehttpc_worker_sup}.
Context: shutdown.
Reason: reached_max_restart_intensity.
Offender: id={whc_http_dm_pool,...}

2026-08-29T10:39:27.555045+08:00 [error]
stacktrace: [{ehttpc,request_async,[false,...]}, ...].
hint: <<"Unknown error: function_clause">>