External integrations can send WhatsApp template messages, but they do
not have a focused API to discover the templates available for an inbox.
This adds an authenticated endpoint that lists the inbox's cached
WhatsApp templates and optionally filters them by exact template name.
Existing inbox responses and template synchronization behavior remain
unchanged.
Fixes https://github.com/chatwoot/chatwoot/issues/13959
### Things to know
- The endpoint returns templates already synchronized and cached for a
direct WhatsApp inbox; it does not make a new provider request.
- Non-WhatsApp inboxes return an unprocessable entity response.
### How to test
1. Authenticate as a user assigned to a WhatsApp inbox.
2. Request `GET
/api/v1/accounts/:account_id/inboxes/:id/message_templates` and verify
the response contains the inbox's templates under `payload`.
3. Repeat the request with `?name=<template_name>` and verify only the
exact matching template is returned.
4. Request the endpoint for a non-WhatsApp inbox and verify it returns
an unprocessable entity response.
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Co-authored-by: Muhsin <12408980+muhsin-k@users.noreply.github.com>
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