* fix: rate-limit pairing rejection messages to prevent spam
When generate_code() returns None (rate limited or max pending), the
"Too many pairing requests" message was sent on every subsequent DM
with no cooldown. A user sending 30 messages would get 30 rejection
replies — reported as potential hack on WhatsApp.
Now check _is_rate_limited() before any pairing response, and record
rate limit after sending a rejection. Subsequent messages from the
same user are silently ignored until the rate limit window expires.
* test: add coverage for pairing response rate limiting
Follow-up to cherry-picked PR #4042 — adds tests verifying:
- Rate-limited users get silently ignored (no response sent)
- Rejection messages record rate limit for subsequent suppression
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Co-authored-by: 0xbyt4 <35742124+0xbyt4@users.noreply.github.com>
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