fix(http): make response callbacks exception-safe

[LimiNode]

Summary

• centralize intermediate and final HTTP response callback invocation in HttpRequestContext
• always run terminal completion after a throwing user callback
• isolate rate-limit and group completion callbacks so one exception cannot skip the other
• route callback failures through the existing NetworkWorker error path
• cover throwing callbacks, continued processing, idempotent cleanup, and waiter registration after shutdown

Verification

• full local integration suite: 13/13 passed
• http_client_wait_requests_test: passed five additional repeated runs
• full_cpp11_smoke: built successfully
• simple_http_request_example: built successfully
• git diff --check: passed