fix: Change contact_inboxes.source_id to text column (#12882)

## Description

Fixes CW-5961 where IMAP email processing failed with
`ActiveRecord::RecordInvalid: Validation failed: Source is too long
(maximum is 255 characters)` error.

This changes the `contact_inboxes.source_id` column from `string` (255
character limit) to `text` (unlimited) to accommodate long email message
IDs that were causing validation failures.

Fixes CW-5961

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)

## How Has This Been Tested?

- Added spec test validating `source_id` values longer than 255
characters (300 chars)
- All existing `contact_inbox_spec.rb` tests pass (7 examples, 0
failures)
- Migration applied successfully with reversible up/down methods
- Verified `source_id` column type changed to `text` with `null: false`
constraint preserved

## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes

app/models/contact_inbox.rb

@@ -9,7 +9,7 @@
 #  updated_at    :datetime         not null
 #  contact_id    :bigint
 #  inbox_id      :bigint
-#  source_id     :string           not null
+#  source_id     :text             not null
 #
 # Indexes
 #

app/models/conversation.rb

@@ -40,6 +40,7 @@
 #  index_conversations_on_contact_inbox_id            (contact_inbox_id)
 #  index_conversations_on_first_reply_created_at      (first_reply_created_at)
 #  index_conversations_on_id_and_account_id           (account_id,id)
+#  index_conversations_on_identifier_and_account_id   (identifier,account_id)
 #  index_conversations_on_inbox_id                    (inbox_id)
 #  index_conversations_on_priority                    (priority)
 #  index_conversations_on_status_and_account_id       (status,account_id)