fix(bitrix24): skip native-group mention gate for Open Channel sessions

[tech-synity]

Problem

Follow-up to #1295. After scoping the Open Channel mention gate by connector, customer messages on 1-to-1 connectors (Facebook Messenger, Zalo OA, …) are still being silently dropped — by the second mention-gate stanza further down in handleMessage, which checks the channel-level c.RequireMention() flag without distinguishing Open Channel traffic from native group chats (CRM deal/task chats).

For a channel like nguyen-dao-openline configured with RequireMention=true, the flow was:

1. isOpenChannel block → shouldRequireMentionForOpenline("facebook|…") returns false (facebook isn't in the whitelist) → no drop.
2. isGroup block (isOpenChannel implies isGroup) → c.RequireMention()=true AND mentioned=false → DROP.

Net effect: the gate-by-connector decision from #1295 was overridden one stanza later, and every Facebook customer turn disappeared. Reproduced live: customer sends "alo" from the Facebook Open Channel, payload arrives at goclaw (mime=audio/mpeg not relevant; same for plain text), and the bot stays silent.

Fix

Scope the drop check inside the isGroup block to native-group chats only (!isOpenChannel). Open Channel sessions already own their mention policy in the earlier isOpenChannel block; re-applying the channel-level RequireMention flag here second-guesses that decision.

if isGroup {
    mentioned := c.isMentionedParams(&evt.Params)
    if !isOpenChannel && c.RequireMention() && !mentioned {  // ← was: if c.RequireMention() && !mentioned
        // drop
        return
    }
    // text processing (MessageOriginal, stripMention, bxConvertUserMentionsToReadable)
    // stays unconditional so Openline staff turns with [USER=<botID>] BBCode still get cleaned up.
}

Importantly, the text-processing that follows the drop check (MessageOriginal fallback, stripMention, bxConvertUserMentionsToReadable) stays unconditional, so an Open Channel staff turn that does come in with [USER=<botID>]Bot[/USER] BBCode still gets cleaned up the same way it always did. The only thing the new guard suppresses is the drop itself.

Verification

go build ./... && go build -tags sqliteonly ./...
go vet ./internal/channels/bitrix24/...
go test ./internal/channels/bitrix24/...   → ok

Live-tested against tamgiac.bitrix24.com after deploying a local image build:

• Facebook Messenger customer turn (IS_CONNECTOR=Y, CHAT_ENTITY_ID=facebook|34|…) now reaches the agent loop. Bot replies, matching the intent from feat(bitrix24): scope Open Channel mention gate to group-style connectors only #1295.
• Zalo personal customer turn (whitelisted connector) without @mention still drops at the earlier gate — no regression.
• Native group chats (CRM deal chats) without @mention still drop here when channel RequireMention=true — no regression.

Surface parity

Backend-only. No API contract / web UI / CLI change.

[B24:2794]

[clark-cant]

Approved. Clean follow-up to #1295 — the second mention-gate stanza was overriding the connector-scoped decision from the upstream gate, silently dropping customer turns on 1-to-1 connectors (Facebook, Zalo OA, etc.) via Open Channel.

The fix is minimal (+11/-1), well-documented, and correctly scopes the drop check to native group chats only while keeping text processing unconditional. CI green, live-verified.

Risk: Low. Backend-only, no API/contract change.