test(gateway): add unit tests for LINE hybrid reply/push logic

[chaodu-agent]

Context

PR #608 introduced the hybrid LINE Reply/Push strategy in gateway/src/main.rs. The gateway currently has zero tests. The new stateful cache and dispatch logic should have unit test coverage.

What to test

Unit tests (no HTTP mocking required)

• Cache size bound — fill cache to REPLY_TOKEN_CACHE_MAX (10k), verify the 10,001st insert is rejected with a warn! and does not panic
• TTL expiry — insert a token, verify it is valid immediately after insert; simulate elapsed time > REPLY_TOKEN_TTL_SECS and verify it is treated as expired
• token_unusable predicate — extract the 400-body string matching into a helper function and test:
  • "Invalid reply token" → token_unusable = true (fall back to Push)
  • "The reply token has expired" → token_unusable = true
  • "Internal server error" (5xx) → token_unusable = false (no fallback, duplicate safety)
  • Unrecognized 400 body → token_unusable = false

Integration tests (requires HTTP mock, follow-up)

• Reply API success → Push API not called
• Reply API returns 400 with token-unusable body → fallback to Push
• Reply API returns 5xx → no fallback (duplicate safety preserved)
• reply_to empty → direct Push (no cache lookup attempted)

Related

• PR feat(gateway): implement hybrid LINE reply/push strategy #608: feat(gateway): implement hybrid LINE reply/push strategy
• PR feat(core): propagate event_id to GatewayReply.reply_to #619: feat(core): propagate event_id to GatewayReply.reply_to

[github-actions]

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