Closes #25721

Problem

LINE webhook endpoint could return 405 Method Not Allowed for HEAD probes. Some webhook validators / reverse proxies / health checks use HEAD instead of GET, which can make the webhook look unhealthy.

Root Cause

createLineNodeWebhookHandler only treated GET as a non-POST verification/probe method. HEAD requests fell through to the generic non-POST branch and returned 405 with Allow: GET, POST.

Fix

• Accept HEAD requests on the LINE webhook path as a lightweight probe response (204 No Content)
• Keep GET probe support unchanged (200 OK)
• Update Allow header to advertise GET, HEAD, POST
• Add regression test coverage for HEAD

Testing

• pnpm exec vitest run src/line/webhook-node.test.ts src/line/webhook.test.ts
• Result: 2 test files passed, 15 tests passed

Greptile Summary

Added support for HEAD requests to the LINE webhook endpoint, resolving #25721 where health checks and webhook validators using HEAD instead of GET would receive a 405 Method Not Allowed response. The implementation correctly returns 204 No Content for HEAD requests (per HTTP spec - no body for HEAD), maintains existing GET support (200 OK), and updates the Allow header to advertise all three supported methods: GET, HEAD, POST. Test coverage includes a regression test for the new HEAD behavior.

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• The change is well-scoped, logically correct, and properly tested. The implementation follows HTTP specifications (204 for HEAD), maintains backward compatibility with existing GET/POST handlers, and includes regression test coverage. The fix addresses a real operational issue without introducing new complexity or risk.
• No files require special attention

_{Last reviewed commit: 3e17498}