feat: Add voice calling as a capability on Twilio SMS channel(Enterprise) (#13963)

Voice calling is now a capability on the existing TwilioSms rather than
a separate Voice model. A single Twilio phone number handles both SMS
and voice calls through one inbox.
Fixes
https://linear.app/chatwoot/issue/CW-6683/add-voice-calling-as-a-capability-on-twilio-sms-channel
and https://linear.app/chatwoot/issue/PLA-120/add-the-support-for-sms

**What changed**
- Replaced Channel::Voice with voice_enabled flag on Channel::TwilioSms
- Added voice_enabled, twiml_app_sid, api_key_secret columns to
channel_twilio_sms table
  - Dropped channel_voice table (no production data)
- All voice logic lives in Enterprise layer via
prepend_mod_with('Channel::TwilioSms')
- Added Voice settings tab on Twilio SMS inbox settings to
enable/disable voice
  - Validates Twilio number voice capability before provisioning
- Teardown service cleans up TwiML app and credentials when voice is
disabled
- Frontend voice detection uses isVoiceCallEnabled() /
getVoiceCallProvider() helpers — extensible to future providers
  - Gated by channel_voice feature flag

**How to test**

1. Enable feature flag:
Account.find(<id>).enable_features('channel_voice')
2. Create voice inbox: Inboxes → Voice tile → enter Twilio credentials →
verify incoming/outgoing calls and SMS work
3. Enable voice on existing SMS inbox: Inboxes → select Twilio SMS inbox
→ Voice tab → toggle on → provide API key credentials → verify calls
work
4. Disable voice: Voice tab → toggle off → verify TwiML app is deleted,
credentials cleared, SMS still works
5. Re-enable voice: Toggle on again → must provide api_key_secret again
→ new TwiML app provisioned

---------

Co-authored-by: Muhsin <12408980+muhsin-k@users.noreply.github.com>

app/javascript/dashboard/featureFlags.js

@@ -36,6 +36,7 @@ export const FEATURE_FLAGS = {
   CHATWOOT_V4: 'chatwoot_v4',
   CHANNEL_INSTAGRAM: 'channel_instagram',
   CHANNEL_TIKTOK: 'channel_tiktok',
+  CHANNEL_VOICE: 'channel_voice',
   CONTACT_CHATWOOT_SUPPORT_TEAM: 'contact_chatwoot_support_team',
   CAPTAIN_CUSTOM_TOOLS: 'custom_tools',
   CAPTAIN_V2: 'captain_integration_v2',