fix: move unreachable event cases out of the message content switch

[kkdai]

The bug

main.go handled four event types as cases on the inner switch:

switch e := event.(type) {
case webhook.MessageEvent:
    switch message := e.Message.(type) {   // <- switch over MessageContentInterface
    case webhook.TextMessageContent:    ...
    case webhook.StickerMessageContent: ...
    case webhook.MemberJoinedEvent:     // never reachable
    case webhook.MemberLeftEvent:       // never reachable
    case webhook.FollowEvent:           // never reachable
    case webhook.BeaconEvent:           // never reachable
    }
}

These are event types, not message contents, so they could never match. Normally the compiler rejects an impossible type switch case, but it can't here: MessageContentInterface is just

type MessageContentInterface interface { GetType() string }

and the event types happen to have a GetType() string too, so they satisfy it structurally. Four silently dead branches in a template people copy from.

A second bug underneath

Moving them to the outer switch is not enough — every case asserted e.Source.(webhook.UserSource), but Source is a SourceInterface whose concrete type depends on the chat. Member joined/left events only occur in group and multi-person chats, so the source is a GroupSource or RoomSource. The unchecked assertion panics, and a panic in an http.HandlerFunc kills the request:

interface conversion: webhook.SourceInterface is webhook.GroupSource, not webhook.UserSource

(reproduced against a real memberJoined payload — see TestOldCodePanics reasoning captured in TestMemberJoinedIsEventNotMessageContent).

The fix

• Move FollowEvent, MemberJoinedEvent, MemberLeftEvent and BeaconEvent to the outer switch over cb.Events
• Add sourceID to render user / group / room sources without asserting a single concrete type
• Log the actual payload: joined/left members come from e.Joined / e.Left, the beacon hwid from e.Beacon — the source alone was never the interesting field
• Guard the optional pointer sub-objects with orZero so a payload that omits them logs instead of nil-derefing
• Fix the copy-pasted "Member joined" log message on the member left case
• Reword the outer default from "Unsupported message: %T" to "Unsupported event: %T", which is what it actually reports

Tests

First tests in this repo. main_test.go decodes a real memberJoined webhook payload and pins down both facts the old code got wrong — that it is an event rather than a message content, and that its source is a GroupSource — plus table tests for sourceID, userIDs and the nil-pointer paths.

• go build ./...
• go vet ./...
• gofmt -l . clean
• go test ./... — 4 tests, all passing

Note

Rebased on top of master after #31 (SDK v8.22.0 + README) was merged, so CI here runs against Go 1.25 and line-bot-sdk-go v8.22.0.