test(gateway): add wiring integration test for LINE hybrid dispatch

[chaodu-agent]

Context

PR #623 added helper-level unit tests for dispatch_line_reply() covering the 6 dispatch decision scenarios. During review, 擺渡法師 identified a gap: there is no test covering the full gateway wiring path.

This is a follow-up from the PR #623 review discussion.

What is missing

A test that exercises the actual gateway wiring:

1. Webhook writes cache — line_webhook handler receives a LINE event with replyToken and inserts event_id → (replyToken, timestamp) into reply_token_cache
2. OAB reply reads cache — A WebSocket reply with reply_to = event_id hits the cache and retrieves the correct replyToken
3. Correct API branch — Verify the reply actually goes through Reply API (with the cached token) or Push API (on cache miss)
4. End-to-end payload correctness — Verify Authorization header, replyToken/to, and message body reach the LINE API

Why this matters

The current unit tests call dispatch_line_reply() directly with a pre-populated cache. If someone breaks the event_id ↔ reply_to wiring between line_webhook and the WebSocket reply handler, the existing 6 tests would still pass green.

Suggested approach

• Start an axum test server with the full router
• POST a LINE webhook payload → verify cache is populated
• Connect a WebSocket client, receive the event, send a reply → verify the correct LINE API is called
• Use wiremock for the LINE API mock

Related

• PR test(gateway): add coverage for hybrid LINE reply/push dispatch #623 (helper-level tests)
• PR feat(gateway): implement hybrid LINE reply/push strategy #608 (hybrid dispatch implementation)
• Issue test(gateway): add coverage for hybrid LINE reply/push dispatch #620 (original test coverage request)

[github-actions]

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