fix: Change messages.source_id to text column (#12908)

## Summary
Changes `messages.source_id` from `string` (255 char limit) to `text`
(20,000 char limit) to support long email Message-ID headers.

## Changes
- Migration to change column type from string to text
- Added spec tests for source_id length validation

## Related
Fixes
https://linear.app/chatwoot/issue/CW-5961/activerecordrecordinvalid-validation-failed-source-is-too-long-maximum

app/models/message.rb

@@ -20,7 +20,7 @@
 #  conversation_id           :integer          not null
 #  inbox_id                  :integer          not null
 #  sender_id                 :bigint
-#  source_id                 :string
+#  source_id                 :text
 #
 # Indexes
 #