fix(gateway): config.yaml path for WhatsApp/Weixin text-batch delays

Convert the salvaged text-debounce delays from HERMES_* env vars to
config.yaml (gateway.platforms.<name>.extra.text_batch_delay_seconds /
text_batch_split_delay_seconds), per the '.env is for secrets only'
policy. Adds a finite/non-negative guard so bad YAML values fall back to
the defaults instead of crashing asyncio.sleep().

- whatsapp.py / weixin.py: read delays via _coerce_float_extra(config.extra)
- update Weixin content-dedup regression test for the deferred dispatch path
- add text-debounce coverage (whatsapp + weixin): defaults, config override,
  bad-value fallback, env-var-ignored, burst-collapse, lone-message
- docs: WhatsApp + Weixin config keys

website/docs/user-guide/messaging/weixin.md

@@ -123,6 +123,8 @@ Set these in `config.yaml` under `platforms.weixin.extra`:
 | `allow_from` | `[]` | User IDs allowed for DMs (when dm_policy=allowlist) |
 | `group_allow_from` | `[]` | Group IDs allowed (when group_policy=allowlist) |
 | `split_multiline_messages` | `false` | When `true`, split multi-line replies into multiple chat messages (legacy behavior). When `false`, keep multi-line replies as one message unless they exceed the length limit. |
+| `text_batch_delay_seconds` | `3.0` | Quiet period (seconds) before a buffered burst of rapid text messages is flushed as one combined request. iLink delivers messages individually, so this debounce avoids one agent invocation per fragment. Set `0` to dispatch each message immediately. |
+| `text_batch_split_delay_seconds` | `5.0` | Extended flush delay used when the latest fragment is near the split threshold (long messages iLink may have chunked). |
 
 ## Access Policies