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

db/migrate/20251119161025_change_messages_source_id_to_text.rb

@@ -0,0 +1,9 @@
+class ChangeMessagesSourceIdToText < ActiveRecord::Migration[7.1]
+  def up
+    change_column :messages, :source_id, :text
+  end
+
+  def down
+    change_column :messages, :source_id, :string
+  end
+end